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                                                                  Exhibit 10.50
                                                                  -------------

                  SCIENTIFIC ADVISORY AND COSULTING AGREEMENT
                  -------------------------------------------

     THIS AGREEMENT, effective as Of August 1, 1994, is by and between Douglas
A. Melton (the "Scientific Advisor"), and Ontogeny, Inc., a Delaware corporation
(the "Company").

     The parties agree as follows:

     1.   Term. The term of the Scientific Advisor's consulting arrangement
hereunder (the "Term") shall commence on the date hereof, and, unless sooner
terminated as provided in this Agreement, shall continue through July 31, 1999,
provided that the Term may be extended by mutual written agreement of the
parties subject to the prior written approval of the Institute. The Term shall
automatically terminate upon the death or permanent disability of the Scientific
Advisor.

     2.   Service as Scientific Advisor and Consultant.

          (a)  The Company shall retain Scientific Advisor as a consultant and
     Chairman of its Scientific Advisory Board, and Scientific Advisor shall
     serve the Company as a consultant and Chairman of its Scientific Advisory
     Board, upon the terms and conditions set forth in this Agreement.

          (b)  During the Term, Scientific Advisor shall render to the Company
     such consulting services in his fields of expertise and knowledge related
     to the business of the Company and at such times and places as the Company
     may from time to time reasonably request, including the strategic direction
     of the Company and future strategic decisions, recruitment and direction of
     key technical personnel, creation of competitive technology assets (such as
     patents and technology sharing agreements), and consultation with the
     Company's technical personnel, research sponsors and collaborators. In
     recognition to the Institution Rules (as defined below) the Company and the
     Consultant agree that the provision of services hereunder may involve the
     exchange of ideas but the consultant shall not direct or conduct research
     for or on behalf of the Company. The parties agree and acknowledge that
     such consulting services shall primarily be rendered at the Company's
     offices in Massachusetts.

          (c)  The Scientific Advisor shall devote, at the Company's request, up
to 20 full days or full-day equivalents to the performance of services hereunder
in each 12 month period (August 1 to July 31) during the Term. A "full-day
equivalent" shall mean eight (8) business hours. The Scientific Advisor shall
not, during the Term, devote any available consulting time to any other
commercial organization, provided, however, that the Scientific Advisor may
devote up to four days per annum to consulting for Gilead Sciences, Inc.

     3.   Certain Other Obligations.

          (a)  The Company recognizes that the Scientific Advisor's primary
     responsibility is to Harvard University and the Howard Hughes Medical
     Institute (the "Institute") (collectively, the "Institutions") and that the
     Scientific Advisor has entered into certain agreements with, and is subject
     to certain policies of, the Institutions relating to the ownership of
     intellectual property, conflicts of interest and similar matters (the
     "Institution Rules"), copies of
<PAGE>

     which has been provided to the Company. If any provisions of this Agreement
     are in conflict with the Institution Rules, the Institution Rules will
     govern and control to the extent of such conflict. The Scientific Advisor
     shall not disclose to the Company any information that (i) the Scientific
     Advisor is obligated to keep secret pursuant to a confidentiality agreement
     with the Institutions or any other third party or (ii) constitutes
     technology, inventions or other intellectual property of the Institutions
     or any other third party.

          (b)  The consulting work performed hereunder will not be conducted on
     time that is required to be devoted to the Institutions. The Scientific
     Advisor shall not use the funding, resources and/or facilities of the
     Institutions or any third party to perform consulting services hereunder
     and shall not perform such consulting services in any manner that would
     give the Institutions or any third party intellectual property rights or
     any other rights to the product of such services.

          (c)  The Scientific Advisor has disclosed, and will disclosure during
     the term of this Agreement, to the Chief Executive Officer of the Company
     any potential conflicts between this Agreement and other contracts binding
     the Scientific Advisor.

     4.   No Competition. Scientific Advisor agrees that, during the Term and
for one (1) year after the end of the Term, Scientific Advisor shall not,
directly or indirectly,

          (a)  as an individual proprietor, partner, stockholder, officer,
     employee, consultant, director, investor, lender or in any other capacity
     (other than as the holder of not more than one percent of the outstanding
     stock of a publicly held company), develop or sell (or assist any other
     person in developing or selling) products or services competitive with
     those developed or sold or planned to be developed or sold, by the Company
     during the Term; or

          (b)  directly or indirectly, solicit or induce any employee of the
     Company to leave the employ of the Company.

     However, the foregoing shall not affect Scientific Advisor's obligations
to, or research on behalf of, the Institute or the University, including,
without limitation, obligations or research of Scientific Advisor in connection
with a transfer by the Institute or the University of materials or intellectual
property developed in whole or in part by Scientific Advisor, or in connection
with research collaborations.

     5.   Compensation.

          (a)  In consideration for the services rendered by Scientific Advisor
     to the Company, the Company shall pay Scientific Advisor, during the Term,
     compensation as follows:

               (1)  a consulting fee of $30,000 per annum, payable in equal
          quarterly installments within 30 days after the end of each three-
          month period during the period from August 1 to July 31 of each year
          (a "Contract Year").

               (2)  a fee of $1,000 for each full day or full-day equivalent of
          consulting services performed hereunder.
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          (b)  In addition, the Scientific Advisor and the Company shall execute
     and deliver a Stock Restriction Agreement, upon terms mutually agreed upon,
     relating to the issuance of shares of Common Stock of the Company to the
     Scientific Advisor. It is agreed that at no time will Scientific Advisor's
     ownership interest in the Company (treating all options as exercised and
     all shares as vested) exceed 5% of the Company's outstanding equity.

          (c)  Scientific Advisor shall not be entitled to any compensation for
     his services or time devoted to the Company in his capacity as a member of
     the Board of Directors of the Company.

     6.   Expenses. The Company shall reimburse Scientific Advisor for his
reasonable out-of-pocket expenses incurred in the performance of his duties
hereunder upon compliance with reasonable administrative policies established
from time to time by the Company.

     7.   Confidentiality. The Scientific Advisor recognizes and acknowledges
that all technology, know-how, inventions and business plans communicated to,
learned of, developed or otherwise acquired by the Scientific Advisor in the
course of his services hereunder and not in the course of his activities as an
institute employee or University faculty member ("Confidential Information") are
valuable assets of the Company to be kept confidential and not disclose or use,
except in connection with the fulfillment of his consulting services for the
company under this Agreement, any Confidential Information of the Company.
"Confidential Information" shall not include, however, information placed in the
public domain through no fault of the Scientific Advisor; information disclosed
to the Scientific Advisor by a third party entitled to disclose it; information
already known to the Scientific Advisor prior to receipt thereof from the
company; or information that is independently developed by the Scientific
Advisor without reference to information provided by the Company and not in the
course of the performance of his services hereunder.

     8.   Representations of the Scientific Advisor. The Scientific Advisor
hereby represents that his current principal employer has received full
disclosure as to the Scientific Advisor's acting as a scientific advisor to the
Company and of the duties required of the Scientific Advisor under this
Agreement, and that such employer, if necessary, has consented to the Scientific
Advisor's execution of the Agreement and position as a scientific advisor for
the Company. The Scientific Advisor further represents that there are no binding
agreements to which he is a party or by which he is bound forbidding or
restricting his activities hereunder, except as disclosed to the Company. In
addition, subject to Section 21 below, the Scientific Advisor consents to being
named as a Scientific Advisor in various reports, brochures or other documents
produced by or on behalf of the Company, including any and all documents filed
with the Securities and Exchange Commission.

     9.   Assignment of Inventions.

          (a)  Subject to Section 9(b) below, the Scientific Advisor hereby
     assigns to the Company all his right, title and interest in and to all
     inventions, discoveries, data, technologies and improvements, whether or
     not patentable or copyrightable, relating to the business or planned
     business of the Company that are made, conceived or reduced to practice,
     alone or jointly with others, in the course of the performance of his
     services hereunder and not in the
<PAGE>

     course of his activities as an Institute employee or University faculty
     member (collectively, "Inventions"), and all patents, copyrights and other
     intellectual property rights relating thereto. The Scientific Advisor
     agrees to cooperate fully in the prosecution of any patent application
     relating to any such Invention, at the expense of the company, which
     cooperation shall include executing any necessary documents in connection
     therewith.

          (b)  The Company shall have no rights by reason of this Agreement in
     any invention, discovery, data, technology or improvement, whether or not
     patentable or copyrightable, which is developed as a result of a program of
     research financed, in whole or in part, by funds provided by or under the
     control of the Institute or the University.

          (c)  The Scientific Advisor shall promptly disclose to the Company all
     Inventions and will maintain adequate and current written records (in the
     form of notes, records, laboratory or research notebooks and as may be
     reasonably specified in advance by the Company) to document the conception
     and/or first actual reduction to practice of any Invention Such written
     records shall be available to and remain the sole property of the Company
     at all times.

     10.  Termination.

          (a)  This Agreement shall terminate upon (i) the death or permanent
     disability of the Scientific Advisor, (ii) the bankruptcy or cessation of
     operations of the Company, (iii) or upon termination for "Cause" (as
     defined below), or (iv) 30 days written notice given by either party to the
     other party. Upon such termination, the Company's obligations to the
     Scientific Advisor hereunder shall terminate, except for the payment of any
     consulting charges accrued and unpaid prior to the date of termination.

          (b)  The term "Cause,, means any of the following: (i) the Scientific
     Advisor's intentional or willful failure to perform his obligations under
     this Agreement in any material respect or the Scientific Advisor's material
     breach of any provision of this Agreement if such failure or breach shall
     continue for 30 days after notice in writing from the Chief Executive
     Officer of the Company specifying such failure or breach; (ii) the
     Scientific Advisor's conviction of a felony; or (iii) the Scientific
     Advisor's conviction of any lesser crime or offense committed in connection
     with the performance of his duties hereunder and involving moral turpitude.
     If the Company notifies the Scientific Advisor in writing that he is to be
     terminated for Cause, the Scientific Advisor shall have the right,
     exercisable within 10 days of receipt of such notice to submit the matter
     to the Board of Directors which will, within 20 day's after the Scientific
     Advisor's written request, meet and make a good faith determination whether
     Cause for termination exists, provided that in making such determination
     the Board of Directors shall give the Scientific Advisor an opportunity to
     appear at the Board meeting and be heard on the matter.

     11.  Survival of Provisions. The provisions of Sections 4.7, 9, 12 and 20
hereof shall survive the termination or expiration of this Agreement in
accordance with their terms.

     12.  Status. Scientific Advisor's relationship to the company shall be that
of an independent contractor and neither this Agreement nor the services to be
rendered hereunder shall for any purpose whatsoever or in any way or manner
create any employer-employee
<PAGE>

relationship between the parties. Scientific Advisor shall not be deemed an
agent for any purpose and shall have no authority to bind the Company in any
way, except as may be specifically authorized by the Company.

     13.  Assignability and Binding Effect. This Agreement shall inure to the
benefit of and shall be binding upon the Company and the Scientific Advisor and
their respective successors and permitted assigns, including any corporation or
entity with which, or into which, the Company may be merged or which may succeed
to its business or assets. The obligations of Scientific Advisor hereunder are
personal, and he may not assign or transfer any of his obligations or rights
hereunder.

     14.  Headings. The paragraph headings contained herein are included solely
for convenience of reference and shall not control or affect the meaning or
interpretation of any of the provisions of this Agreement.

     15.  Notices. Any notices or other communication hereunder by either party
shall be in writing and shall be deemed to have been duly given if delivered
personally to the other party or sent by registered or certified mail, return
receipt requested, to the other party at the following addresses:

          If to the Company:

               Ontogeny, Inc.
               One Kendall Square
               Building 600
               Cambridge, MA 02139

          If to the Scientific Advisor:

               Douglas A. Melton
               22 Slocum Road
               Lexington, MA 02173

or at such other address as such other party may designate in conformity with
the foregoing.

     16.  Entire Agreement. This Agreement constitutes the entire agreement
between the parties and supersedes all prior agreements and understandings,
whether written or oral, relating to the subject matter of this Agreement.

     17.  Delays or Omissions. No delay or omission by the Company in exercising
any right under this Agreement shall operate as a waiver of that or any other
right. A waiver or consent given by the Company on any one occasion shall be
effective only in that instance and shall not be construed as a bar or waiver of
any right on any other occasion.

     18.  Governing Law. This Agreement shall be governed by, and construed and
enforced in accordance with, the laws of the Commonwealth of Massachusetts
applicable to contracts made and to be performed therein, without giving effect
to the principles thereof relating to the conflict of laws.
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     19.  Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

     20.  Indemnity. The Company agrees, at its sole expense, to defend the
Institute and Scientific Advisor, and to indemnify and hold the Institute and
Scientific Advisor harmless from, any claims or suits by a third party against
the Institute or Scientific Advisor or any liabilities or judgments based
thereon, either arising from Scientific Advisor's performance of services for
the Company under this Agreement or arising from any Company products which
result from Scientific Advisor's performance of services under this Agreement,
provided that the Company shall have the right to control the defense and
settlement of such claims or suits.

     21.  Use of Names. The Company will not use Scientific Advisor's or the
Institute's or the University's name in any commercial advertisement or similar
material that is used to promote or sell products, unless the company obtains in
advance the written consent of the named party to such use, in addition, if the
Scientific Advisor's name is to be used for such purposes, the consent of the
Institute must also be obtained.

     22.  Consent. The Company and Scientific Advisor acknowledge that any
amendment of this Agreement (including, without limitation, any extension of
this Agreement or any change in the consideration to be provided to Scientific
Advisor with respect to services to be provided hereunder) or any departure from
the terms or conditions hereof with respect to Scientific Advisor's consulting
services for the Company is subject to the institute's prior written approval.

     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement as
of the day and year first above written.

                              ONTOGENY, INC.

                              By:  /s/ William Helman
                                 --------------------
                                   President

                              SCIENTIFIC ADVISOR

                              /s/ Douglas A. Melton
                              ---------------------
                                  Douglas A. Melton
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ONTOGENY, INC.
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November 12,1997


Dr. Douglas A. Melton
Dept. of Molecular & Cellular Biology
Harvard University
7 Divinity Avenue
Cambridge, MA 02138

Dear Doug,

Ontogeny would like to simplify your compensation (eliminating the need to bill
for the additional $1,000 per day of consulting services), by amending Section 5
as follows:

     Subparagraphs (a) and (b)
     ---------------------------
     Delete both subparagraphs, and insert "(a) pay to the Consultant an annual
     fee of $50,000 during the term hereof, payable in equal quarterly
     installments of $12,500 within thirty (30) days after the end of each
     three-month period from August 1 to July 31 of each year, and (b)".

     Subparagraph (c) becomes subparagraph (b).
     ----------------

All other terms of your Agreement with Ontogeny shall remain in full force and
effect.

If you are in agreement with the above, please sign the enclosed copy of this
letter and send us one copy for our files.

Very truly yours,


George A. Eldridge
Vice President, Finance             Acknowledged and Agreed,



                                    /s/ Douglas A. Melton
                                    -----------------------------------
                                    Douglas A. Melton
                                    Date:  /s/ 20 Nov. 97
                                          -----------------------------
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ONTOGENY, INC.
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January 11, 2000


Douglas Melton, Ph.D.
Dept. of Molecular and Cellular Biology
Harvard University
Howard Hughes Medical Institute
7 Divinity Avenue
Cambridge, MA 02138

Dear Doug:

As part of a legal audit, we recently found that your Scientific Advisory and
Consulting Agreement does not contain an automatic renewal like our other SAB
agreements, but instead contains a five year term ending July 31,1999, with an
option to extend by mutual written agreement.  We have been operating under the
premise that you are still a current member of our Scientific Advisory Board and
would propose to amend your Agreement effective as of August 1, 1994 to add an
automatic renewal clause as follows:

In Section 1, lines 4 and 5, please delete, "provided that the Term may be
extended by mutual written agreement of the parties subject to the prior written
approval of the Institute".  Please add the following at the end of Section 1:
"The Term will be automatically renewed for successive one-year periods, unless
either party gives written notice of no-renewal to the other at least 30 days
prior to the expiration of the initial, or then current renewal term."

All other terms of your Agreement with Ontogeny shall remain in full force and
effect.

If you are in agreement with the above, please sign the enclosed copy of this
letter and send us one copy for our files.  Please call me if you have any
questions or concerns.  We apologize for the oversight and for any
inconvenience.

Very truly yours,

Raul Rodriguez
Senior Vice President, Business Development and Operations

Acknowledged and Agreed,

/s/ Douglas Melton
---------------------------------------
Douglas Melton, Ph.D.

Date:  /s/ 22 Jan.  00
       --------------------------------