Document:

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                                                                     Exhibit 4.1

                                                                    COMMON STOCK

NUMBER                                                                    SHARES

___________________                                            _________________

                            AmeriVest Properties Inc.

                                             SEE REVERSE FOR CERTAIN DEFINITIONS

                            INCORPORATED UNDER THE LAWS OF THE STATE OF MARYLAND
 THIS CERTIFICATE IS TRANSFERABLE IN THE CITIES OF KANSAS CITY, MO, OR NEW YORK,
                                                                             NY

This Certifies that ___________________________

is the record holder of ___________________________

   FULLY PAID AND NONASSESSABLE SHARES OF THE COMMON STOCK, $.001 PAR VALUE OF

    ------------------------AmeriVest Properties Inc.------------------------

transferable on the books of the Corporation by the Holders hereof in person or
by duly authorized attorney upon surrender of this Certificate properly
endorsed. This Certificate is not valid until countersigned by the Transfer
Agent and registered by the Registrar.

WITNESS the Seal of the Corporation and the facsimile signatures of its duly
authorized officers.

Dated

/s/ Alexander S. Hewitt                /s/ Charles K. Knight
-------------------------         ------------------------------
Secretary                         President

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  KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN, OR DESTROYED THE
CORPORATION WILL REQUIRE A BOND OF INDEMNITY AS A CONDITION TO THE ISSUANCE OF A
REPLACEMENT CERTIFICATE.

  The following abbreviations, when used in the inscription on the face of this
certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

  TEN COM  -  as tenants in common         UNIF GIFT MIN ACT ____ Custodian ____
  TEN ENT  -  as tenants by the entireties                   (  )        (Minor)
  JT TEN   -  as joint tenants with right          under Uniform Gifts to Minors
              of survivorship and not as           Act _______________
              tenants in common                           (State)

                                           UNIF TRF MIN ACT ________ Custodian
                                                                (until age ___)
                                                 _______ under Uniform Transfers
                                                 (Minor)
                                           to Minors Act _______________________
                                                               (State)

     Additional abbreviations may also be used though not in the above list.

         FOR VALUE RECEIVED, ___________________________ hereby sell, assign and
transfer unto

  PLEASE INSERT SOCIAL SECURITY OR OTHER
  IDENTIFYING NUMBER OF ASSIGNEE

  ____________________________________________________________________________
  (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE OF ASSIGNEE)

  ____________________________________________________________________________

  ____________________________________________________________________________

  _______________________________________________________________________Shares
  of the common stock represented by the within Certificate, and do hereby
  irrevocably constitute and appoint ________
  ___________________________________________________________________Attorney to
  transfer the said stock on the books of the within named Corporation with full
  power of substitution in the premises.

  Dated _______________________

                                        X________________________________

                                        X________________________________
                                        THE SIGNATURE(S) TO THIS ASSIGNMENT MUST
                                        CORRESPOND WITH THE NAME(S) AS WRITTEN
                                NOTICE: UPON THE FACE OF THE CERTIFICATE IN
                                        EVERY PARTICULAR, WITHOUT ALTERNATION OR
                                        ENLARGEMENT OR ANY CHANGE WHATEVER.

  Signature(s) Guaranteed

  By ________________________________________________________________
  THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTOR
  INSTITUTION (BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND
  CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE
  MEDALLION PROGRAM, PURSUANT TO S.E.C. RULE 17Ad-15.

The Company's Articles of Incorporation and Bylaws, as amended, set forth a full
statement of (a) the designations and any preferences, conversion and other
rights, voting powers, restrictions, limitations as to dividends,
qualifications, and terms and conditions of redemption of the stock of each
class of shares which the Company is authorized to issue and (b) the authority
of the Board of Directors to issue any preferred or special class in series, the
differences in the relative rights and preferences between the shares of stock
of each series to the extent they have been set and the authority of the Board
of Directors to set the relative rights and preferences of subsequent series of
preferred stock. The Company will furnish a copy of such statement to any
stockholder on request and without charge.

The shares of common stock evidenced by this Certificate are subject to
restrictions on ownership and transfer as set forth in the Company's Articles of
Incorporation or Bylaws, as amended, for purposes of the Company's maintenance
of its status as a real estate investment trust under the Internal Revenue Code
of 1986, as amended. Any transfer of shares of stock in violation of these
ownership and transfer restrictions will result in the shares of stock being
designated and treated as "Excess Shares" which shall be held in trust by an
unaffiliated trustee for the benefit of a charitable beneficiary. The Company
will furnish a copy of such ownership and transfer restrictions to any
stockholder on request and without charge.Exhibit 10.55

                    AMENDMENT TO LICENSE AND OPTION AGREEMENT
                                (the "Amendment")

     This  Amendment  is  effective  as of the 8th day of  April,  2002,  by and
     between H.C. Implants B.V., a corporation duly organized and existing under
     the laws of the Netherlands, having its registered office at Zernikedreef 6
     (2333 CL) Leiden,  the Netherlands,  and/or Osteotech,  Inc., a corporation
     duly  organized  and  existing  under the laws of the State of New  Jersey,
     having its registered  office at 51 James Way, Eaton Town, New Jersey,  USA
     hereinafter jointly  "Licensor",  and IsoTis N.V., formerly known as Matrix
     Medical  Holding B.V., a corporation  duly organized and existing under the
     laws of the  Netherlands,  having its  statutory  seat in Amsterdam and its
     office  at  Professor   Bronkhorstlaan  10  D  (3723  MB)  Bilthoven,   the
     Netherlands, hereinafter "Licensee",

     WHEREAS:

          A.   Licensor  and  Licensee  have  entered  into a License and Option
               Agreement  on the  27th  day of  June,  1997,  of which a copy is
               attached to this Amendment, hereinafter the "Agreement";

          B.   Capitalized terms used in this Amendment refer to the capitalized
               terms and definitions that have been used in the Agreement;

          C.   Licensee is desirous to exercise its option  under the  Agreement
               to purchase  the  Patents and  Licensor is desirous to assign the
               Patents to Licensee;

          D.   Licensee is desirous to enter into an  agreement  to acquire from
               Cam Implants B.V., a wholly owned subsidiary of Licensor ("CAM"),
               its   business,   certain  of  its  assets  and  certain  of  its
               liabilities (the "CAM  Business"),  hereinafter the "CAM Business
               Acquisition  Agreement."  Licensee  and  CAM  have  entered  into
               negotiations  to that effect and they intend to transfer  the CAM
               Business no later than May 31st, 2002;

          E.   Licensee and Licensor  wish to amend the  Agreement to the effect
               that the  payment by  Licensee  and the  execution  of the patent
               assignment by Licensor is subject to the terms and  conditions as
               set out in the Agreement and this Amendment thereto;

          F.   Following the execution and  implementation of this Amendment the
               Agreement  will  be  deemed  to  have  been  terminated  save  as
               otherwise provided here below.

     NOW THEREFORE:

     Licensor and Licensee agree that:

          1.   Section  2.3 of the  Agreement  will be  amended so as to read as
               follows:

               "2.3 (a) So long as Licensee  is not in default of any  provision
               of the  Agreement,  Licensee will have the option to purchase the
               Patents (the "Purchase  Option") for a purchase price equal to an
               initial payment of USD 1,000,000 ("Initial Payment").

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                    (b) An  additional  USD  250,000  will be due by Licensee to
               Licensor in accordance with the following conditions:

                    (i)  if  Licensee  falls  to  enter  into  the CAM  Business
                         Acquisition  Agreement  on or prior to May 31st,  2002,
                         Licensee will pay an  additional  amount of USD 250,000
                         to Licensor no later than May 31st, 2002; or

                    (ii) if  Licensee  decides  prior to May  31st,  2002 not to
                         enter into the CAM Business Acquisition  Agreement,  an
                         additional  amount  of USD  250,000  will  be  due  and
                         payable  to  Licensor  within  5  business  days  after
                         informing CAM of such  decision,  but no later than May
                         31st, 2002.

                    (c)  Section  2.3  (b)(i)  of  the  Agreement  will  not  be
               applicable, in the event that, on or prior to May 31st, 2002, CAM
               has not made  available to Licensee such  information  concerning
               the  CAM  Business  which  Licensee  may  reasonably  require  in
               relation to its proposed  acquisition of the CAM Business as long
               as such information actually exists, or In the event that CAM has
               decided  on or before  May 31st,  2002 not to enter  into the CAM
               Business Acquisition Agreement.

                    (d) The Purchase Option shall be exercised by written notice
               given by Licensee to the  Licensor,  which notice is deemed to be
               given  through  the  signing of this  Amendment,  and  payment by
               Licensee to Licensor of an amount of USD  1,250,000  (the "Escrow
               Amount") no later than April 10th,  2002 by means of a telephonic
               transfer by Licensee to bank account number 41.18.32.352 with ABN
               Amro Bank N.V. in the name of Stibbe Derdengelden Nortariaat (the
               "Escrow Account").

                    (e) Licensor  shall execute a patent  assignment in the form
               set forth in Appendix B,  forthwith  upon payment by the Licensee
               of the Escrow Amount into the Escrow Account.

                    (f) An amount of USD  1,000,000  of the Escrow  Amount  plus
               accrued  interest will be released to Licensor upon the following
               conditions having been fulfilled:

                        (i)   the due execution by the Licensor and the Licensee
                              of the Patent Assignment in the form of Appendix B
                              hereto and the  signing by Licensor of any further
                              forms or deeds  required  to be signed or executed
                              by Licensor in  connection  with the  requisite IP
                              filings  at the  relevant  patent  and trade  mark
                              offices in and  outside  the  Netherlands,  all as
                              listed in Appendix C hereto, and

                        (ii)  delivery  to  the  Licensee  of  all  intellectual
                              property files in relation to the Patents.

                                        2
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          It is understood  that Licensee will be responsible for and undertakes
          to do  whatever  is  necessary  to do any  filings  with the Office of
          Patents and Trademarks of the Netherlands, the US Patent and Trademark
          Office  and any  corresponding  offices  of  countries  foreign to the
          Netherlands  and the  USA at its  costs,  and  that  Licensor  will do
          whatever it can to assist the Licensee therein if requested to do so.

                           (g) An amount of USD  250,000  of the  Escrow  Amount
                  plus accrued  interest  will be released to Licensor  upon the
                  occurrence of a situation as described in section 2.3 (b),

                           (h) An amount of USD  250,000  of the  Escrow  Amount
                  plus  accrued  interest  will be  released  to  Licensee  upon
                  Licensor  and  Licensee  having  entered into the CAM Business
                  Acquisition Agreement on or before May 31st, 2002, or upon the
                  occurrence of a situation as described in section 2.3 (c).

          2.   Section  6.1(b)(i)  of the  Agreement  will be  deleted  from the
               Agreement,  and  Section  6.1(b)(ii)  of the  Agreement  will  be
               re-numbered to Section 6.1(b)(i).

          3.   The first  sentence of Section  6.1(b)(iii) of the Agreement will
               be  deleted  from the  Agreement,  and  Section  (b)(iii)  of the
               Agreement will be re-numbered to Section 6.1(b)(ii).

          4.   Section 1.4 of the Agreement  will be amended on the date hereof,
               so as to read as follows:

               "1.4 The term  "Patents"  shall  mean all valid  and  enforceable
               claims of patents and patent  applications  related to Polyactive
               owned by or controlled by Licensor,  which are filed prior to the
               execution date of the patent assignment (in the form set forth in
               Appendix B hereto) by Licensor  pursuant  to the  exercise of the
               Purchase Option by Licensee.  The Patents,  patent  applications,
               and short  particulars  of which shall be described in Appendix A
               hereto".

          5.   Section 12.9 will be added to the  Agreement and will contain the
               following wording:

               "Licensee will sell to the Licensor,  at its request,  polyactive
               material  and/or  polyactive  material  product at prices no less
               favourable  then the best price offered to any other  customer of
               the Licensee for such quantities,  for three years as of the date
               hereof."

          6.   Appendix  A to the  Agreement  will be  replaced  by the  amended
               Appendix A, of which a copy is attached to this Amendment.

          7.   Appendix  B to the  Agreement  will be  replaced  by the  amended
               Appendix B, of which a copy is attached to this Amendment.

          8.   Licensor  confirms  that  there  is no  default  by  Licensee  as
               referred to in Section 2.3 of the Agreement as hereby amended.

                                        3
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          9.   Following the execution and  implementation  of this Amendment it
               is  understood  that the  Agreement  will be  deemed to have been
               terminated   with   the   exception   of   Section   Sections   4
               ("Confidentiality"),  8.2  ("Warranties"),  11.8 (no  release  of
               obligations  matured prior to termination),  12.2  (jurisdiction)
               and 12.4 (applicable law).

          10.  HC  Implants  B.V.  and  Osteotech,  Inc.,  each in  capacity  as
               Licensor,  warrant and represent,  both severally and jointly, to
               Licensee  that (i) the Patents are in full force and effect,  are
               vested in and are legally and beneficially  owned by the relevant
               Licensor,  are valid and  enforceable,  and none of them is being
               used, claimed,  opposed or attached by any other person, and (ii)
               no right or license has been  granted to any third  party,  other
               than to  Licensee,  to use in any manner or to do anything  which
               would or might  otherwise  infringe any of the Patents and no act
               has been done,  omitted or permitted  by Licensor  whereby any of
               the  Patents   have  ceased  or  might  cease  to  be  valid  and
               enforceable.

     IN WITNESS  WHEREOF,  the  Licensor  and the Licensee  have  executed  this
     Amendment as of the date set forth above.

     -----------------------------------     -----------------------------------
     H.C. Implants B.V.                      ISOTis N.V.
     By:                                     By:
     Date:                                   Date:
     At:                                     At:

     -----------------------------------
     Osteotech, Inc.
     By:
     Date:
     At:

     Annexes to this Amendment:

     1.    License and Option Agreement of 27 June 1997
     2.    Appendix A (List of Patents)
     3.    Appendix B (Patent Assignment)
     4.    Appendix C (Assignment registration forms)

                                        4

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