<PAGE>

                                                                   Exhibit 10.61

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "1933 ACT"). THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED, OR
HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT RELATED
THERETO OR AN OPINION OF COUNSEL (WHICH MAY BE COMEPANY COUNSEL) REASONABLY
SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE
1933 ACT.

                              WARRANT AGREEMEENT

                  To Purchase 5,000 Shares of Common Stock of

                                ONTOGENY, INC.

             Dated as of December 17, 1999 (the "Effective Date")

     WHEREAS, Ontogeny, Inc., a Delaware corporation (the "Company") has entered
into a Master Equipment Lease Agreement dated as of December 17, 1999, Lease
Line Schedule No. 01, and certain other related schedules and other documents
(collectively, as amended or supplemented from time to time the "Leases") with
Lighthouse Capital III, L.P., a Delaware limited partnership (the
"Warrantholder"); and

     WHEREAS, the Company desires to grant to Warrantholder, in consideration
for such Leases, the right to purchase shares of its Common Stock;

     NOW, THEREFORE, in consideration of the Warrantholder executing and
delivering such Leases and in consideration of the mutual covenants and
agreements contained herein, the Company and Warrantholder agree as follows:

1.   GRANT OF THE RIGHT TO PURCHASE COMMON STOCK.

     The Company hereby grants to the Warrantholder, and the Warrantholder is
entitled, upon the terms and subject to the conditions hereinafter set forth, to
subscribe to and purchase, from the Company, at a price per share equal to $5.00
(the "Exercise Price"), 5,000 fully paid and nonassessable shares of the
Company's Common Stock, $.01 par value per share (the "Common Stock"). The
number and purchase price of such shares are subject to adjustment as provided
in Section 8 hereof.

2.   TERM OF THE WARRANT AGREEMENT.

     (a)  Except as otherwise provided for herein, the term of this Warrant
Agreement and the right to purchase Common Stock as granted herein shall
commence on the Effective Date and shall be exercisable for a period of ten (10)
years.

     (b)  Acceleration of Term Upon Initial Public Offering. Notwithstanding the
term of this Warrant Agreement fixed pursuant to Section 2(a) hereof,
Warrantholder's right to purchase Common Stock as granted herein shall expire,
if not previously exercised, immediately upon the closing of the issuance and
sale of shares of Common Stock of the Company in the Company's first public
offering of securities for its own account pursuant to an effective
<PAGE>

registration statement under the 1933 Act (the "Initial Public Offering"),
provided that the underwriters request that the Warrantholder exercise this
Warrant.

     The Company shall notify the Warrantholder if the Initial Public Offering
is proposed, within a reasonable period of time prior to the filing of a
registration statement and if the Company fails to deliver such written notice
within a reasonable period of time, anything to the contrary in this Warrant
Agreement notwithstanding, the rights to purchase will not expire until ten (10)
business days after the Company delivers such notice to the Warrantholder. Such
notice shall also contain such details of the proposed Initial Public Offering
as are reasonable in the circumstances and notice that this Warrant Agreement is
expected to expire upon the later of (i) the closing of the Initial Public
Offering or (ii) the date which is ten (10) days after the date the
Warrantholder receives the notice. If such closing does not take place within
six (6) months, the Company shall promptly notify the Warrantholder that such
proposed transaction has been terminated. Anything to the contrary in this
Warrant Agreement notwithstanding, the Warrantholder may rescind any exercise of
its purchase rights promptly after such notice of termination of the proposed
transaction if the exercise of Warrants occurred after the Company notified the
Warrantholder that the Initial Public Offering was proposed or if the exercise
were otherwise precipitated by such proposed Initial Public Offering. In the
event of such rescission, the Warrants will continue to be exercisable on the
same terms and conditions.

3.   EXERCISE OF THE PURCHASE RIGHTS.

     The purchase rights set forth in this Warrant Agreement are exercisable by
the Warrantholder, in whole or in part, at any time, or from time to time, prior
to the expiration of the term set forth in Section 2 above, by tendering to the
Company at its principal office a notice of exercise in the form attached hereto
as Exhibit I (the "Notice of Exercise"), duly completed and executed. Promptly
upon receipt of the Notice of Exercise and the payment of the purchase price in
accordance with the terms set forth below, and in no event later than twenty-one
(21) days thereafter, the Company shall issue to the Warrantholder a certificate
for the number of shares of Common Stock purchased and shall execute the Notice
of Exercise indicating the number of shares which remain subject to future
purchases, if any.

     The Exercise Price may be paid at the Warrantholder's election either (i)
by cash or check, (ii) by the surrender by the Warrantholder to the Company of
any promissory notes or other obligations issued by the Company, with all such
notes and obligations so surrendered being credited against the Exercise Price
in an amount equal to the principal amount thereof plus accrued interest to the
date of surrender, (iii) by surrender of Warrants ("Net Issuance") as determined
below, or (iv) by any combination of the foregoing. If the Warrantholder elects
the Net Issuance method, the Company will issue Common Stock in accordance with
the following formula:


                    X   =    Y(A-B)
                             ------
                                A

Where:              X   =    the number of shares of Common Stock to be issued
                             to the Warrantholder.

                                      -2-
<PAGE>

          Y =  the number of shares of Common Stock requested
               to be exercised under this Warrant Agreement.

          A =  the fair market value of one (1) share of
               Common Stock.

          B =  the Exercise Price in effect under the Warrant
               Agreement at the time the net issue election is
               made pursuant to this Section 3.

     As used herein, current fair market value of a share of Common Stock shall
mean with respect to each share of Common Stock:

          (i)   if the exercise is in connection with an initial public
offering, and if the Company's Registration Statement relating to such public
offering has been declared effective by the SEC, then the initial "Price to
Public" specified in the final prospectus with respect to the offering;

          (ii)  if this Warrant is exercised after, and not in connection with
the Company's initial public offering, and:

                (1) if traded on a securities exchange, the fair market value of
the Common Stock shall be deemed to be the average of the closing prices over a
twenty-one (21 ) day period ending three days before the day the current fair
market value of the securities is being determined and the fair market value of
the Common Stock shall be deemed to be such fair market value of the Common
Stock;

                (2) if actively traded over-the-counter, the fair market value
of the Common Stock shall be deemed to be the average of the closing bid and
asked prices quoted on the NASDAQ system (or similar system) over the twenty-one
(21) day period ending three days before the day the current fair market value
of the securities is being determined and the fair market value of the Common
Stock shall be deemed to be such fair market value of the Common Stock; and

          (iii) if at any time the Common Stock is not listed on any securities
exchange or quoted in the NASDAQ System or the over-the-counter market, the
current fair market value of the Common Stock shall be the highest price per
share which the Company could obtain from a willing buyer (not a current
employee or director) for shares of Common Stock sold by the Company, from
authorized but unissued shares, as determined in good faith by its Board of
Directors, and the fair market value of the Common Stock shall be deemed to be
such fair market value of the Common Stock; provided, that if the Company shall
become subject to a merger, acquisition or other consolidation pursuant to which
the Company is not the surviving party, the fair market value of the Common
Stock shall be deemed to be the value per share of the consideration received by
the holders of the Company's Common Stock pursuant to such merger or acquisition
and the fair market value of the Common Stock shall be deemed to be such fair
market value of the Common Stock.

     Upon partial exercise by either cash or Net Issuance, the Company shall
promptly issue an amended Warrant Agreement representing the remaining number of
shares purchasable

                                      -3-
<PAGE>

hereunder. All other terms and conditions of such amended Warrant Agreement
shall be identical to those contained herein, including, but not limited to the
Effective Date hereof

4.   RESERVATION OF SHARES.

     (a)  Authorization and Reservation of Shares. During the term of this
Warrant Agreement, the Company will at all times have authorized and reserved a
sufficient number of shares of its Common Stock to provide for the exercise of
the rights to purchase Common Stock as provided for herein.

     (b)  Registration or Listing. If any shares of Common Stock required to be
reserved hereunder require registration with or approval of any governmental
authority under any Federal or State law (other than any registration under the
1933 Act, as then in effect, or any similar Federal statute then enforced, or
any state securities law, required by reason of any transfer involved in such
conversion), or listing on any domestic securities exchange, before such shares
may be issued upon conversion, the Company will, at its expense and as
expeditiously as possible, use its best efforts to cause such shares to be duly
registered, listed or approved for listing on such domestic securities exchange,
as the case may be.

5.   NO FRACTIONAL SHARES OR SCRIP.

     No fractional shares or scrip representing fractional shares shall be
issued upon the exercise of the Warrant, but in lieu of such fractional shares
the Company shall make a cash payment therefor upon the basis of the Exercise
Price then in effect.

6.   NO RIGHTS AS SHAREHOLDER.

     This Warrant Agreement does not entitle the Warrantholder to any voting
rights or other rights as a shareholder of the Company prior to the exercise of
the Warrant.

7.   WARRANTHOLDER REGISTRY.

     The Company shall maintain a registry showing the name and address of the
registered holder of this Warrant Agreement.

8.   ADJUSTMENT RIGHTS.

     The purchase price per share and the number of shares of Common Stock
purchasable hereunder are subject to adjustment, as follows:

     (a)  Merger and Sale of Assets. If at any time there shall be a capital
reorganization of the shares of the Company's stock (other than a combination,
reclassification, exchange or subdivision of shares otherwise provided for
herein), or a merger or consolidation of the Company with or into another
corporation when the Company is not the surviving corporation, or the sale of
all or substantially all of the Company's properties and assets to any other
person (hereinafter referred to as a "Merger Event"), then, as a part of such
Merger Event lawful provision shall be made so that the Warrantholder shall
thereafter be entitled to receive, upon exercise of the Warrant, the number and
kind of shares of stock or other securities of the

                                      -4-
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successor corporation or cash or other property that the Warrantholder would
have been entitled to receive if such Warrantholder had exercised this Warrant
immediately prior to the Merger Event. In any such case, appropriate adjustment
(as determined in good faith by the Company's Board of Directors) shall be made
in the application of the provisions of this Warrant Agreement with respect to
the rights and interest of the Warrantholder after the Merger Event to the end
that the provisions of this Warrant Agreement (including adjustments of the
Exercise Price and number of shares of Common Stock purchasable) shall be
applicable to the greatest extent possible.

     (b)  Reclassification of Shares.  If the Company at any time shall, by
combination, reclassification, exchange or subdivision of securities or
otherwise, change any of the Common Stock into the same or a different number of
securities of any other class or classes, this Warrant Agreement shall
thereafter represent the right to acquire such number and kind of securities as
would have been issuable as the result of such change with respect to the
securities which were subject to the purchase rights under this Warrant
Agreement immediately prior to such combination, reclassification, exchange,
subdivision or other change.

     (c)  Subdivision or Combination of Shares. If the Company at any time shall
combine or subdivide (by split-up or otherwise) its Common Stock or issue
additional shares of Common Stock in payment of a stock dividend on the Common
Stock, the number of shares of Common Stock issuable on the exercise of this
Warrant Agreement shall forthwith be proportionately increased in the case of a
subdivision or stock dividend, or proportionately decreased in the case of a
combination, and the Exercise Price shall be proportionately decreased in the
case of a subdivision or stock dividend, or proportionately increased in the
case of a combination.

     (d)  Antidilution Rights. Additional antidilution rights applicable to the
Common Stock purchasable hereunder are as set forth in the Company's Certificate
of Incorporation, as amended through the Effective Date, a true and complete
copy of which is attached hereto as Exhibit III (the "Charter"). The Company
shall promptly provide the Warrantholder with any restatement, amendment,
modification or waiver of the Charter affecting such anti-dilution rights. The
Company will give Warrantholder notices of all dilutive issuances of its stock.

     (e)  Notice of Adjustments. If: (i) the Company shall declare any dividend
or distribution upon its stock, whether in cash, property, stock or other
securities; (ii) the Company shall offer for subscription pro rata to the
holders of any class of its Preferred Stock or other convertible stock any
additional shares of stock of any class or other rights; (iii) there shall be
any Merger Event; or (iv) there shall be any voluntary or involuntary
dissolution, liquidation or winding up of the Company; then, in connection with
each such event, the Company shall send to the Warrantholder: (A) at least
twenty (20) days prior written notice of the date on which the books of the
Company shall close or a record shall be taken for such dividend, distribution,
subscription rights (specifying the date on which the holders of Common Stock
shall be entitled thereto) or for determining rights to vote in respect of such
Merger Event, dissolution, liquidation or winding up; and (B) in the case of any
such Merger Event, dissolution, liquidation or winding up, at least twenty (20)
days prior written notice of the date when the same shall take place (and
specifying the date on which the holders of Common Stock shall be entitled to
exchange their

                                      -5-
<PAGE>

Common Stock for securities or other property deliverable upon such Merger
Event, dissolution, liquidation or winding up).

     Each such written notice shall set forth, in reasonable detail, (i) the
event requiring the adjustment, (ii) the amount of the adjustment, (iii) the
method by which such adjustment was calculated, (iv) the Exercise Price, and (v)
the number of shares subject to purchase hereunder after giving effect to such
adjustment, and shall be given by first class mail, postage prepaid, addressed
to the Warrantholder, at the address as shown on the books of the Company.

     (f)  Timely Notice. Failure to timely provide such notice required by
subsection (e) above shall entitle Warrantholder to retain the benefit of the
applicable notice period notwithstanding anything to the contrary contained in
any insufficient notice received by Warrantholder. The notice period shall begin
on the date Warrantholder actually receives a written notice containing all the
information specified above.

9.   REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY.

     (a)  Reservation of Common Stock. The Common Stock issuable upon exercise
of the Warrantholder's rights and the Common Stock issuable upon conversion
thereof has been duly and validly reserved and, when issued in accordance with
the provisions of this Warrant Agreement, will be validly issued, fully paid and
non-assessable, and will be free of any taxes, liens, charges or encumbrances of
any nature whatsoever; provided, however, that the Common Stock issuable
pursuant to this Warrant Agreement may be subject to restrictions on transfer
under state and/or Federal securities laws. The Company has made available to
the Warrantholder true, correct and complete copies of its Charter and Bylaws,
as amended, and minutes of all Board of Directors (including all committees of
the Board of Directors, if any) and Shareholder meetings from inception through
September 8, 1997. The issuance of certificates for shares of Common Stock upon
exercise of the Warrant shall be made without charge to the Warrantholder for
any issuance tax in respect thereof, or other cost incurred by the Company in
connection with such exercise and the related issuance of shares of Common
Stock. The Company shall not be required to pay any tax which may be payable in
respect of any transfer involved and the issuance and delivery of any
certificate in a name other than that of the Warrantholder.

     (b)  Due Authority. The execution and delivery by the Company of this
Warrant Agreement and the performance of all obligations of the Company
hereunder, including the issuance to Warrantholder of the right to acquire the
shares of Common Stock, have been duly authorized by all necessary corporate
action on the part of the Company, and the Leases and this Warrant Agreement are
not inconsistent with the Company's Charter or Bylaws, do not contravene any law
or governmental rule, regulation or order applicable to it, do not and will not
contravene any provision of, or constitute a default under, any indenture,
mortgage, contract or other instrument to which it is a party or by which it is
bound, and the Leases and this Warrant Agreement constitute legal, valid and
binding agreements of the Company, enforceable in accordance with their
respective terms.

     (c)  Consents and Approvals. No consent or approval of, giving of notice
to, registration with, or taking of any other action in respect of any state,
Federal or other

                                      -6-
<PAGE>

governmental authority or agency is required with respect to the execution,
delivery and performance by the Company of its obligations under this Warrant
Agreement, except for the filing of notices pursuant to Regulation D under the
1933 Act and Section 25102(f) of the California Corporate Securities Law, which
filings will be effective by the time required thereby.

     (d)  Issued Securities.  All issued and outstanding shares of Common Stock,
Preferred Stock or any other securities of the Company have been duly authorized
and validly issued and are fully paid and nonassessable.  All outstanding shares
of Common Stock, Preferred Stock and any other securities were issued in full
compliance with all Federal and state securities laws.  In addition:

          (i)    The authorized capital stock of the Company consists of (A)
50,000,000 shares of common stock, $.01 par value per share (the "Common
Stock"), 2,844,765 shares of which are issued and outstanding (including
1,487,535 shares issued pursuant to the 1994 Restricted Stock Plan of the
Company), (B) 5,000,000 shares of undesignated preferred stock, none of which
are issued or outstanding, (C) 4,922,299 shares of Series A Convertible
Preferred Stock, 4,853,334 shares of which are issued and outstanding, (D)
8,000,000 shares of Series B Convertible Preferred Stock, 7,449,223 shares of
which are issued and outstanding, (E) 1,800,000 shares, collectively, of Series
C Convertible Preferred Stock, Series C1 Convertible Preferred Stock, and Series
D Convertible Preferred Stock, all of which are issued and outstanding, (G)
10,000,000 shares of Series E Convertible Preferred Stock, all of which are
issued or outstanding, (H) 10,000,000 Series F Convertible Preferred Stock,
8,379,593 of which are issued and outstanding, (I) 400,000 Series G Convertible
Preferred Stock, 400,000 of which are issued and outstanding.

          (ii)   The Company has issued warrants to purchase 68,962 shares of
its Series A Convertible Preferred Stock at an average exercise price of $0.87
per share and 142,222 shares of its Series B Convertible Preferred Stock at an
average exercise price of $1.125 per share. The Company has reserved 7,494,770
shares of Common Stock for issuance pursuant to its 1995 and 1997 Stock Option
Plans. The Company has reserved 1,487,535 shares of Common Stock for issuance
pursuant to its 1994 Restricted Stock Plan of which 1,487,535 shares are issued
and outstanding.

          (iii)  Except as stated in the Company's Charter and the Company's
Amended and Restated Registration Rights and Right of First Refusal Agreement,
no shareholder of the Company has preemptive rights to purchase new issuances of
the Company's capital stock.

     (e)  Insurance.  The Company has in full force and effect insurance
policies, with extended coverage, insuring the Company and its property and
business against such losses and risks, and in such amounts, as are customary
for corporations engaged in a similar business and similarly situated.

     (f)  Other Commitments to Register Securities.  Except as set forth in this
Warrant Agreement, the Amended and Restated Registration Rights and Right of
First Refusal Agreement dated as of March 12, 1997 between the Company and
certain shareholders and the Stock Purchase Agreement dated September 2, 1994
between the Company and a certain Institution, the Company is not pursuant to
the terms of any other agreement currently in

                                      -7-
<PAGE>

existence, under any obligation to register under the 1933 Act any of its
presently outstanding securities or any of its securities which may hereafter be
issued.

     (g)  Exempt Transaction. Subject to the accuracy of the Warrantholder's
representations in Section 10 hereof, the issuance of the Common Stock upon
exercise of this Warrant will constitute a transaction exempt from (i) the
registration requirements of Section 5 of the 1933 Act, in reliance upon Section
4(2) thereof, and (ii) the qualification requirements of the California
Corporate Securities Law, in reliance upon Section 25102(f) thereof.

     (h)  Compliance with Rule 144. At the written request of the Warrantholder,
who proposes to sell Common Stock issuable upon the exercise of the Warrant in
compliance with Rule 144 promulgated by the Securities and Exchange Commission,
the Company shall furnish to the Warrantholder, within ten days after receipt of
such request, a written statement confirming the Company's compliance with the
filing requirements of the Securities and Exchange Commission as set forth in
such Rule, as such Rule may be amended from time to time.

10.  REPRESENTATIONS AND COVENANTS OF THE WARRANTHOLDER.

     This Warrant Agreement has been entered into by the Company in reliance
upon the following representations and covenants of the Warrantholder:

     (a)  Investment Purpose. The right to acquire Common Stock or the Common
Stock issuable upon exercise of the Warrantholder's rights contained herein will
be acquired for investment and not with a view to the sale or distribution of
any part thereof, and the Warrantholder has no present intention of selling or
engaging in any public distribution of the same except pursuant to a
registration or exemption.

     (b)  Private Issue. The Warrantholder understands (i) that the Common Stock
issuable upon exercise of this Warrant is not registered under the 1933 Act or
qualified under applicable state securities laws on the ground that the issuance
contemplated by this Warrant Agreement will be exempt from the registration and
qualifications requirements thereof, and (ii) that the Company's reliance on
such exemption is predicated on the representations set forth in this Section
10.

     (c)  Disposition of Warrantholder's Rights. In no event will the
Warrantholder make a disposition of any of its rights to acquire Common Stock or
Common stock issuable upon exercise of such rights unless and until (i) it shall
have notified the Company of the proposed disposition, and (ii) if requested by
the Company, it shall have furnished the Company with an opinion of counsel
(which counsel may either be inside or outside counsel to the Warrantholder)
satisfactory to the Company and its counsel to the effect that (A) appropriate
action necessary for compliance with the 1933 Act has been taken, or (B) an
exemption from the registration requirements of the 1933 Act is available.
Notwithstanding the foregoing, the restrictions imposed on the transferability
of any of its rights to acquire Common Stock or Common Stock issuable on the
exercise of such rights do not apply to transfers from the beneficial owner of
any of the aforementioned securities to its nominee or from such nominee to its
beneficial owner, and shall terminate as to any particular share of Common Stock
when (1) such security shall have

                                      -8-
<PAGE>

been effectively registered under the 1993 Act and sold by the holder thereof in
accordance with such registration or (2) such security shall have been sold
without registration in compliance with Rule 144 under the 1933 Act, or (3) a
letter shall have been issued to the Warrantholder at its request by the staff
of the Securities and Exchange Commission stating that no action shall be
recommended by such staff or taken by such Commission, as the case may be, if
such security is transferred without registration under the 1933 Act in
accordance with the conditions set forth in such letter or ruling and such
letter or ruling specifies that no subsequent restrictions on transfer are
required. Whenever the restrictions imposed hereunder shall terminate, as
hereinabove provided, the Warrantholder or holder of a share of Common Stock
then outstanding as to which such restrictions have terminated shall be entitled
to receive from the Company, without expense to such holder, one or more new
certificates for the Warrant or for such shares of Common Stock not bearing any
restrictive legend.

     Notwithstanding anything to the contrary contained herein:

          (i)   The Warrantholder shall not sell or transfer or agree to sell or
transfer any rights to acquire shares of stock of the Company or any shares of
stock of the Company (collectively, the "Offered Shares"), unless in each case
the Warrantholder shall have first complied with this Section 10(c). The
Warrantholder shall deliver to the Company a written notice of any proposed or
intended sale of Offered Shares (the "Offer"), which Offer shall (a) identify
the Offered Shares, (b) describe the price and other terms upon which they are
to be sold or transferred, and the number or amount of the Offered Shares to be
sold or transferred, and (c) offer to sell to the Company all (but not less than
all) such Offered Shares. The Company shall have the right, for a period of five
(5) days following delivery of the Offer, to purchase or acquire, at a price and
upon the other terms specified in the Offer, any and all Offered Shares
described above. The Offer by its term shall remain open and irrevocable for
such 5-day period.

          (ii)  To accept an Offer, the Company must deliver a written notice to
the Warrantholder prior to the end of the 5-day period of the Offer (the "Notice
of Acceptance").  If the Company does not accept the Offer, the Warrantholder
may sell the Offered Shares on the terms and conditions set forth in the Offer
for a period of forty-five (45) days following the end of the 5-day period of
the Offer.

          (iii) Any Offered Shares not acquired by the Company or other persons
in accordance with this Section 10(c) may not be issued, sold or exchanged until
they are again offered to the Company under the procedures specified in this
Agreement.

          (iv)  The provisions of this Section 10(c) shall terminate upon the
closing of the Company's initial public offering of shares of Common Stock
pursuant to an effective registration statement under the Securities Act.

     (d)  Financial Risk. The Warrantholder has such knowledge and experience in
financial and business matters and in investing in companies similar to the
Company as to be capable of evaluating the merits and risks of its investment,
and has the ability to bear the economic risks of its investment.

                                      -9-
<PAGE>

     (e)  Authority. The Warrantholder has full power and authority to enter
into and to perform this Agreement in accordance with its terms. The
Warrantholder has not been organized, reorganized or recapitalized specifically
for the purpose of investing in the Company.

     (f)  Risk of No Registration. The Warrantholder understands that if the
Company does not register with the Securities and Exchange Commission pursuant
to Section 12 of the 1933 Act or file reports pursuant to Section 15(d), of the
Securities Exchange Act of 1934 (the "1934 Act"), or if a registration statement
covering the securities under the 1933 Act is not in effect when it desires to
sell (i) the rights to purchase Common Stock pursuant to this Warrant Agreement,
or (ii) the Common Stock issuable upon exercise of the right to purchase, it may
be required to hold such securities for an indefinite period. The Warrantholder
also understands that any sale of its rights of the Warrantholder to purchase
Common Stock or Common Stock which might be made by it in reliance upon Rule 144
under the 1933 Act may be made only in accordance with the terms and conditions
of that Rule.

11.  TRANSFERS.

     (a) Subject to the terms and conditions contained in Section 10 hereof,
this Warrant Agreement and all rights hereunder are transferable in whole or in
part by the Warrantholder and any successor transferee, provided, however, in no
event shall the number of transfers of the rights and interests in all of the
Warrants exceed three (3) transfers.  The transfer shall be recorded on the
books of the Company upon receipt by the Company of a notice of transfer in the
form attached hereto as Exhibit II (the "Transfer Notice"), at its principal
offices and the payment to the Company of all transfer taxes and other
governmental charges imposed on such transfer.

     (b) Legends.  Each certificate or agreement representing rights to acquire
shares of Common Stock shall bear a legend substantially in the following form:

         "The securities represented by this certificate have
         not been registered under the Securities Act of 1933,
         as amended, and may not be offered, sold or otherwise
         transferred, pledged or hypothecated unless and until
         such shares are registered under such Act, or an
         opinion of counsel satisfactory to the Company is
         obtained to the effect that such registration is not
         required."

     The foregoing legend shall be removed from the certificates representing
any Restricted Shares, at the request of the holder thereof, at such time as
they become eligible for resale pursuant to Rule 144 under the Securities Act.

     (c) If any of the provisions set forth in this Agreement are violated, the
Company shall not be required (i) to transfer on its books any of the rights to
acquire shares of Common Stock, or (ii) to treat as owner of such rights to
acquire shares of Common Stock or to pay dividends to any transferee to whom any
such rights to acquire shares of Common Stock shall have been so sold or
transferred.

                                      -10-
<PAGE>

12.  MISCELLANEOUS.

     (a)  Effective Date.  The provisions of this Warrant Agreement shall be
construed and shall be given effect in all respects as if it had been executed
and delivered by the Company on the date hereof.  This Warrant Agreement shall
be binding upon any successors or assigns of the Company.

     (b)  Attorney's Fees. In any litigation, arbitration or court proceeding
between the Company and the Warrantholder relating hereto, the prevailing party
shall be entitled to attorneys' fees and expenses and all costs of proceeding
incurred in enforcing this Warrant Agreement.

     (c)  Governing Law. This Warrant Agreement shall be governed by and
construed for all purposes under and in accordance with the laws of the State of
California.

     (d)  Counterparts. This Warrant Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

     (e)  Notices. Any notice required or permitted hereunder shall be given in
writing and shall be deemed effectively given upon personal delivery, facsimile
transmission (provided that the original is sent by personal delivery or mail as
hereinafter set forth) or seven (7) days after deposit in the United States
mail, by registered or certified mail, addressed (i) to the Warrantholder at 100
Drakes Landing Road, Suite 260, Greenbrae, CA 94904 with a copy to William S.
Veatch, Esq., c/o Cooley, Godward, Castro, Huddleson & Tatum, Five Palo Alto
Square, 3000 El Camino Real, Palo Alto, CA 94306-2155 and (ii) to the Company at
45 Moulton Street, Cambridge, MA 02138, (and/or if by facsimile, (617) 876-
0866), c/o Chief Financial Officer or at such other address as any such party
may subsequently designate by written notice to the other party.

     (f)  Remedies.  In the event of any default hereunder, the non-defaulting
party may proceed to protect and enforce its rights either by suit in equity
and/or by action at law, including but not limited to an action for damages as a
result of any such default, and/or an action for specific performance for any
default where Warrantholder will not have an adequate remedy at law and where
damages will not be readily ascertainable.

     (g)  No Impairment of Rights.  The Company will not, by amendment of its
Charter or through any other means, avoid or seek to avoid the observance or
performance of any of the terms of this Warrant, but will at all times in good
faith assist in the carrying out of all such terms and in the taking of all such
actions as may be necessary or appropriate in order to protect the rights of the
Warrantholder against impairment.  Any and all costs incurred by the Company in
assisting the Warrantholder in the protection of the Warrantholder's rights
shall be paid for, or reimbursed, by the Warrantholder.

     (h)  Survival. The representations, warranties, covenants and conditions of
the respective parties contained herein or made pursuant to this Warrant
Agreement shall survive the execution and delivery of this Warrant Agreement.

                                      -11-
<PAGE>

     (i)  Severability.  In the event any one or more of the provisions of this
Warrant Agreement shall for any reason be held invalid, illegal or
unenforceable, the remaining provisions of this Warrant Agreement shall be
unimpaired, and the invalid, illegal or unenforceable provision shall be replace
by a mutually acceptable valid, legal and enforceable provision, which comes
closest to the intention of the parties underlying the invalid, illegal or
unenforceable provision.

     (j)  Amendments.  Any provision of this Warrant Agreement may be amended by
a written instrument signed by the Company and by the Warrantholder.

     (k)  Additional Documents.  The Company, upon execution of this Warrant
Agreement, shall provide the Warrantholder with certified resolutions with
respect to the representations, warranties and covenants set forth in
subparagraphs (a) through (d), (f) and (g) of Section 9 above.  The Company
shall also supply such other documents as the Warrantholder may from time to
time reasonably request.

     IN WITNESS WHEREOF, the parties hereto have cause this Warrant Agreement to
be executed by its officers thereunto duly authorized as of the Effective Date.

                              COMPANY:


                              ONTOGENY, INC.


                              By: /s/ George A. Eldridge
                                  ------------------------------------

                              Name:  George A. Eldridge
                              Title: Vice President, Finance


                              WARRANTHOLDER:
                              LIGHTHOUSE CAPITAL PARTNERS III, L.P.

                              By:  LIGHTHOUSE MANAGEMENT
                                   PARTNERS III, L.L.C., its general partner


                              By: /s/ Gwill E. York
                                  ------------------------------------

                              Name:  Gwill E. York

                              Title: Managing Director

                                      -12-
<PAGE>

                                   EXHIBIT I

                              NOTICE OF EXERCISE

To:_______________________________

(1)  The undersigned Warrantholder hereby elects to purchase ______ shares of
     the Common Stock of ___________ pursuant to the terms of the Warrant
     Agreement dated the _______________ day of __________,19_____ (the "Warrant
     Agreement") between _______________ and the Warrantholder, and tenders
     herewith payment of the purchase price for such shares in full.

(2)  In exercising its rights to purchase the Common Stock of _________________,
     the undersigned hereby confirms and acknowledges the investment
     representations and warranties made in Section 10 of the Warrant Agreement.

(3)  Please issue a certificate or certificates representing said shares of
     Common Stock in the name of the undersigned or in such other name as is
     specified below.


__________________________________
(Name)


__________________________________
(Address)


Warrantholder:

By:_______________________________

Title:____________________________

Date:_____________________________

                                      -13-
<PAGE>

                          ACKNOWLEDGMENT OF EXERCISE

     The undersigned __________________, hereby acknowledge receipt of the
"Notice of Exercise" from __________________ to purchase __________ shares of
the Common Stock of _______________ pursuant to the terms of the Warrant
Agreement, and further acknowledges that ____________ shares remain subject to
purchase under the terms of the Warrant Agreement.

                              Company:



                              By:_________________________________

                              Title:______________________________

                              Date:_______________________________

                                      -14-
<PAGE>

                           NET ISSUE ELECTION NOTICE

To:_____________________________________    Date:______________________

     The undersigned hereby elects under Section 3 to surrender the right to
purchase _______ shares   of Common Stock pursuant to this Warrant Agreement.
The certificate(s) for such shares issuable upon such net issue election shall
be issued in the name of the undersigned or as otherwise indicated below:


                              _____________________________________
                              Signature

                              _____________________________________
                              Name for Registration

                              _____________________________________
                              Mailing Address

                                      -15-
<PAGE>

                                  EXHIBIT II

                                TRANSFER NOTICE

     (To transfer or assign the foregoing Warrant Agreement execute this form
     and supply required information.  Do not use this form to purchase shares.)

     FOR VALUE RECEIVED, the foregoing Warrant Agreement and all rights
evidenced thereby are hereby transferred and assigned to

_______________________________________________
                                    (Please Print)

___________________ whose address is _____________________________________
______________________________________________________________________.

                              Dated:______________________________________


                              Holder's Signature:_________________________

                              Holder's Address:___________________________

                              ____________________________________________

                                      -16-
<PAGE>

                                  EXHIBIT III

                   Copy of the Certificate of Incorporation


                                      -17-