ENDOGEN, INC.

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                                     BY-LAWS

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                                    ARTICLE I

                                  Stockholders


      1. Annual Meeting. The annual meeting of stockholders shall be held on the
first Thursday of November in each year (or if that be a legal holiday in the
place where the meeting is to be held, on the next succeeding full business day)
at 10:00 A.M., or at such other date and time as shall be designated from time
to time by the Board of Directors or President. The purposes for which the
annual meeting is to be held, in addition to those prescribed by law, by the
Articles of Organization or by these By-laws, may be specified by the Directors
or the President. In the event an annual meeting has not been held on the date
fixed in these By-laws, a special meeting in lieu of the annual meeting may be
held with all the force and effect of an annual meeting.

      2. Special Meetings. Special meetings of stockholders may be called by the
President or by the Directors. Upon written application of one or more
stockholders who hold at least 10% of the capital stock entitled to vote at a
meeting, a special meeting shall be called by the Clerk, or in case of the
death, absence, incapacity or refusal of the Clerk, by any other officer.

      3. Place of Meetings. All meetings of stockholders shall be held at the
principal office of the corporation unless a different place (within or without
Massachusetts, but within the United States) is fixed by the Directors or the
President and stated in the notice of the meeting.

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      4. Notice of Meetings. A written notice of the place, date and hour of all
meetings of stockholders stating the purpose of the meeting shall be given by
the Clerk or an Assistant Clerk or by the person calling the meeting at least
seven days before the meeting or such longer period as is required by law to
each stockholder entitled to vote thereat and to each stockholder who under the
law, under the Articles of Organization or under these By-laws, is entitled to
such notice, by leaving such notice with him or at his residence or usual place
of business, or by mailing it, postage prepaid, and addressed to such
stockholder at his address as it appears in the records of the corporation.
Whenever notice of a meeting is required to be given a stockholder under any
provision of the Massachusetts Business Corporation Law or of the Articles of
Organization or these By-laws, a written waiver thereof, executed before or
after the meeting by such stockholder or his attorney thereunto authorized and
filed with the records of the meeting, shall be deemed equivalent to such
notice.

      5. Quorum. The holders of a majority in interest of all stock issued,
outstanding and entitled to vote at a meeting shall constitute a quorum, but a
lesser number may adjourn any meeting from time to time without further notice;
except that, if two or more classes of stock are outstanding and entitled to
vote as separate classes, then in the case of each such class, a quorum shall
consist of the holders of a majority in interest of the stock of that class
issued, outstanding and entitled to vote.

      6. Voting and Proxies. Each stockholder shall have one vote for each share
of stock entitled to vote owned by him and a proportionate vote for a fractional
share, unless otherwise provided by the Articles of Organization in the case
that the corporation has two or more classes or series of stock. Capital stock
shall not be voted if any installment of the subscription therefor has been duly
demanded in accordance with the law of the Commonwealth of Massachusetts and is
overdue and unpaid. Stockholders may vote either in person or by written proxy.
Proxies shall be filed with the clerk of the meeting, or of any adjournment
thereof, before being voted. No proxy dated more than six months before the date
named therein shall be valid and no proxy shall be valid after the final
adjournment of such meeting. Notwithstanding the provisions of the 

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preceding sentence, a proxy coupled with an interest sufficient in law to
support an irrevocable power, including, without limitation, an interest in
shares or in the corporation generally, may be made irrevocable if it so
provides, need not specify the meeting to which it relates, and shall be valid
and enforceable until the interest terminates, or for such shorter period as may
be specified in the proxy. Except as otherwise limited therein, proxies shall
entitle the persons named therein to vote at any adjournment of such meeting but
shall not be valid after final adjournment of such meeting. A proxy with respect
to stock held in the name of two or more persons shall be valid if executed by
any one of them unless at or prior to exercise of the proxy the corporation
receives a specific written notice to the contrary from any one of them. A proxy
purporting to be executed by or on behalf of a stockholder shall be deemed valid
unless challenged at or prior to its exercise and the burden of proving
invalidity shall rest on the challenger.

      7. Action at Meeting. When a quorum is present, the holders of a majority
of the stock present or represented and voting on a matter (or if there are two
or more classes of stock entitled to vote as separate classes, then in the case
of each such class, the holders of a majority of the stock of that class present
or represented and voting on a matter), except where a larger vote is required
by law, the Articles of Organization or these By-laws, shall decide any matter
to be voted on by the stockholders. Any election of directors or officers by the
stockholders shall be determined by a plurality of the votes cast by
stockholders entitled to vote at the election. Any such elections shall be by
ballot if so requested by any stockholder entitled to vote thereon. The
corporation shall not directly or indirectly vote any share of its own stock.

      8. Action without Meeting. Any action required or permitted to be taken at
any meeting of the stockholders may be taken without a meeting if all
stockholders entitled to vote on the matter consent to the action in writing and
the written consents are filed with the records of the meetings of stockholders.
Such consent shall be treated for all purposes as a vote at a meeting.

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                                   ARTICLE II

                                    Directors

      1. Powers. The business of the corporation shall be managed by a Board of
Directors who may exercise all the powers of the corporation except as otherwise
provided by law, by the Articles of Organization or by these By-laws. In the
event of vacancy in the Board of Directors, the remaining Directors, except as
otherwise provided by law, may exercise the powers of the full Board until the
vacancy is filled.

      2. Election. A Board of Directors shall be elected by the stockholders at
the annual meeting. The number of directors shall be fixed by the stockholders
(except as that number may be enlarged by the Board of Directors acting pursuant
to Section 4 of this Article), but shall be not less than three, except that
whenever there shall be only two stockholders the number of directors shall be
not less than two and whenever there shall be only one stockholder or prior to
the issuance of any stock, there shall be at least one director, and shall be
not more than nine. Unless changed by the stockholders or directors in
accordance with this Article, the number of directors that constitutes the
entire Board of Directors shall be seven.

      3. Vacancies. Any vacancy in the Board of Directors, however occurring,
including a vacancy resulting from the enlargement of the Board, may be filled
by the stockholders or, in the absence of stockholder action, by the Directors.

      4. Enlargement of the Board. The Board of Directors may be enlarged by the
stockholders at any meeting or by vote of a majority of the Directors then in
office.

      5. Tenure. Except as otherwise provided by law, by the Articles of
Organization or by these By-laws, Directors shall hold office until the next
annual meeting of stockholders and until their successors are chosen and
qualified. Any Director may resign by delivering his written resignation to the
corporation at its principal office or to the President, Clerk or Secretary.
Such resignation shall be effective upon receipt unless it is specified to be
effective at some other time or upon the happening of some other event.

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      6. Removal. A Director may be removed from office (a) with or without
cause by the vote of the holders of a majority of the shares entitled to vote in
the election of Directors, provided that the Directors of a class elected by a
particular class of stockholders may be removed only by the vote of the holders
of a majority of the shares of the particular class of stockholders entitled to
vote for the election of such Directors; or (b) for cause by vote of a majority
of the Directors then in office. A Director may be removed for cause only after
a reasonable notice and opportunity to be heard before the body proposing to
remove him.

      7. Meetings. The directors shall meet at least once during each fiscal
quarter of the corporation. Regular meetings of the Directors may be held
without call or notice at such places and at such times as the Directors may
from time to time determine, provided that any Director who is absent when such
determination is made shall be given notice of the determination. A regular
meeting of the Directors may be held without a call or notice at the same place
as the annual meeting of stockholders.

      Special meetings of the Directors may be held at any time and place
designated in a call by the President, by two or more Directors, or by the
holder or holders of at least 25% of the outstanding shares of Series A
Convertible Preferred Stock.

      8. Telephone Conference Meetings. Members of the Board of Directors may
participate in a meeting of the board by means of a conference telephone or
similar communications equipment by means of which all persons participating in
the meeting can hear each other at the same time and participation by such means
shall constitute presence in person at a meeting.

      9. Notice of Meetings. Notice of all special meetings of the Directors
shall be given to each Director by the Secretary, or Assistant Secretary, or if
there be no Secretary or Assistant Secretary, by the Clerk, or Assistant Clerk,
or in case of the death, absence, incapacity or refusal of such persons, by the
officer or one of the Directors calling the meeting. Notice shall be given to
each Director in person or by telephone, by facsimile transmission or by
telegram sent to his business or home address at least twenty-four hours in
advance of the meeting, or (with respect 

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to Directors whose principal residence is within the continental United States
only) by written notice mailed to his business or home address at least
forty-eight hours in advance of the meeting. Notice of a meeting need not be
given to any Director if a written waiver of notice, executed by him before or
after the meeting, is filed with the records of the meeting, or to any Director
who attends the meeting without protesting prior thereto or at its commencement
the lack of notice to him. A notice or waiver of notice of a Directors' meeting
need not specify the purposes of the meeting.

      10. Quorum. At any meeting of the Directors, a majority of the Directors
then in office shall constitute a quorum. Less than a quorum may adjourn any
meeting from time to time without further notice.

      11. Action at Meeting. At any meeting of the Directors at which a quorum
is present, a majority of the Directors present may take any action on behalf of
the Board except to the extent that a larger number is required by law or the
Articles of Organization or these By-laws.

      12. Action by Consent. Any action required or permitted to be taken at any
meeting of the Directors may be taken without a meeting, if all the Directors
consent to the action in writing and the written consents are filed with the
records of the meetings of Directors. Such consents shall be treated for all
purposes as a vote at a meeting.

      13. Committees. The Directors may, by vote of a majority of the Directors
then in office, elect from their number an executive or other committees and may
by like vote delegate thereto some or all of their powers except those which by
law, the Articles of Organization or these By-laws they are prohibited from
delegating to such committee. Except as the Directors may otherwise determine,
any such committee may make rules for the conduct of its business, but unless
otherwise provided by the Directors or in such rules, its business shall be
conducted as nearly as may be in the same manner as is provided by these By-laws
for the Directors.

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                                   ARTICLE III

                                    Officers

      1. Enumeration. The officers of the corporation shall consist of a
President, a Treasurer, a Clerk, and such other officers, including a Chairman
of the Board of Directors, one or more Vice-Presidents, Assistant Treasurers,
Assistant Clerks, Secretary and Assistant Secretaries as the Directors may
determine.

      2. Election. The President, Treasurer and Clerk shall be elected annually
by the Directors at their first meeting following the annual meeting of
stockholders. Other officers may be chosen by the Directors at such meeting or
at any other meeting.

      3. Qualification. The President may, but need not be, a Director. No
officer need be a stockholder. Any two or more offices may be held by the same
person, provided that the President and Clerk shall not be the same person. The
Clerk shall be a resident of Massachusetts unless the corporation has a resident
agent appointed for the purpose of service of process. Any officer may be
required by the Directors to give bond for the faithful performance of his
duties to the corporation in such amount and with such sureties as the Directors
may determine.

      4. Tenure. Except as otherwise provided by law, by the Articles of
Organization or by these By-laws, the President, Treasurer and Clerk shall hold
office until the first meeting of the Directors following the next annual
meeting of stockholders and until their successors are chosen and qualified; and
all other officers shall hold office until the first meeting of the Directors
following the next annual meeting of stockholders and until their successors are
chosen and qualified, unless a shorter term is specified in the vote choosing or
appointing them. Any officer may resign by delivering his written resignation to
the corporation at its principal office or to the President, Clerk or Secretary,
and such resignation shall be effective upon receipt unless it is specified to
be effective at some other time or upon the happening of some other event.

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      5. Removal. The Directors may remove any officer with or without cause by
vote of a majority of the Directors then in office; provided, that an officer
may be removed for cause only after a reasonable notice and opportunity to be
heard before the Board of Directors.

      6. President, Chairman of the Board, and Vice-President. The President
shall, unless otherwise provided by the Directors, be the chief executive
officer of the corporation and shall, subject to the direction of the Directors,
have general supervision and control of its business. Unless otherwise provided
by the Directors he shall preside, when present, at all meetings of stockholders
and, unless a Chairman of the Board has been elected and is present, of the
Directors.

      If a Chairman of the Board of Directors is elected he shall preside at all
meetings of the Board of Directors at which he is present. The Chairman shall
have such other powers as the Directors may from time to time designate.

      Any Vice-President shall have such powers as the Directors may from time
to time designate.

      7. Treasurer and Assistant Treasurer. The Treasurer shall, subject to the
direction of the Directors, have general charge of the financial affairs of the
corporation and shall cause accurate books of account to be kept. He shall have
custody of all funds, securities, and valuable documents of the corporation,
except as the Directors may otherwise provide.

      Any Assistant Treasurer shall have such powers as the Directors may from
time to time designate.

      8. Clerk and Assistant Clerks. The Clerk shall record all proceedings of
the stockholders in a book to be kept therefor. Unless a transfer agent is
appointed, the Clerk shall keep or cause to be kept in Massachusetts, at the
principal office of the corporation or at his office, the stock and transfer
records of the corporation, in which are contained the names of all stockholders
and the record address and the amount of stock held by each.

      In case a Secretary is not elected, the Clerk shall record all proceedings
of the Directors in a book to be kept therefor.

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      In the absence of the Clerk from any meeting of the stockholders, an
Assistant Clerk, if one be elected, otherwise a Temporary Clerk designated by
the person presiding at the meeting, shall perform the duties of the Clerk. Any
Assistant Clerk shall have such additional powers as the Directors may from time
to time designate.

      9. Secretary and Assistant Secretaries. If a Secretary is elected, he
shall keep a record of the meetings of the Directors and in his absence, an
Assistant Secretary, if one be elected, otherwise a Temporary Secretary
designated by the person presiding at the meeting, shall keep a record of the
meetings of the Directors.

      Any Assistant Secretary shall have such additional powers as the Directors
may from time to time designate.

      10. Other Powers and Duties. Each officer shall, subject to these By-laws,
have in addition to the duties and powers specifically set forth in these
By-laws, such duties and powers as are customarily incident to his office, and
such duties and powers as the Directors may from time to time designate.

                                   ARTICLE IV

                                  Capital Stock

      1. Certificates of Stock. Subject to the provisions of Section 2 below,
each stockholder shall be entitled to a certificate of the capital stock of the
corporation in such form as may be prescribed from time to time by the
Directors. The certificate shall be signed by the President or a Vice-President,
and by the Treasurer or an Assistant Treasurer; provided, however, such
signatures may be facsimiles if the certificate is signed by a transfer agent,
or by a registrar, other than a Director, officer or employee of the
corporation. In case any officer who has signed or whose facsimile signature has
been placed on such certificate shall have ceased to be such officer before such
certificate is issued, it may be issued by the corporation with the same effect
as if he were such officer at the time of its issue.

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      Every certificate issued for shares of stock at a time when such shares
are subject to any restriction on transfer pursuant to the Articles of
Organization, these By-laws or any agreement to which the corporation is a party
shall have the restriction noted conspicuously on the certificate and shall also
set forth on the face or back of the certificate either the full text of the
restriction or a statement of the existence of such restriction and a statement
that the corporation will furnish a copy thereof to the holder of such
certificate upon written request and without charge. Every stock certificate
issued by the corporation at a time when it is authorized to issue more than one
class or series of stock shall set forth upon the face or back of the
certificate either the full text of the preferences, voting powers,
qualifications and special and relative rights of the shares of each class and
series, if any, authorized to be issued, as set forth in the Articles of
Organization, or a statement of the existence of such preferences, powers,
qualifications, and rights, and a statement that the corporation will furnish a
copy thereof to the holder of such certificate upon written request and without
charge.

      2. Stockholder Open Accounts. The corporation may maintain or caused to be
maintained stockholder open accounts in which may be recorded all stockholders'
ownership of stock and all changes therein. Certificates need not be issued for
shares so recorded in a stockholder open account unless requested by the
stockholder.

      3. Transfers. Subject to the restrictions, if any, stated or noted on the
stock certificates, shares of stock may be transferred in the records of the
corporation by the surrender to the corporation or its transfer agent of the
certificate therefor, properly endorsed or accompanied by a written assignment
and power of attorney properly executed, with necessary transfer stamps affixed,
and with such proof of the authenticity of signature as the corporation or its
transfer agent may reasonably require. When such stock certificates are thus
properly surrendered to the corporation or its transfer agent, the corporation
or transfer agent shall cause the records of the corporation to reflect the
transfer of the shares of stock. Except as may be otherwise required by law, by
the Articles of Organization or by these By-laws, the corporation shall be
entitled to treat the record holder of stock as shown in its records as the
owner of such 

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stock for all purposes, including the payment of dividends and the right to vote
with respect thereof, regardless of any transfer, pledge or other disposition of
such stock, until the shares have been transferred on the books of the
corporation in accordance with the requirements of these By-laws.

      It shall be the duty of each stockholder to notify the corporation of his
post office address.

      4. Record Date. The Directors may fix in advance a time which shall be not
more than sixty (60) days before the date of any meeting of stockholders or the
date for the payment of any dividend or the making of any distribution to
stockholders or the last day on which the consent or dissent of stockholders may
be effectively expressed for any purpose, as the record date for determining the
stockholders having the right to notice of and to vote at such meeting and any
adjournment thereof or the right to receive such dividend or distribution or the
right to give such consent or dissent. In such case only stockholders of record
on such record date shall have such right, notwithstanding any transfer of stock
on the books of the corporation after the record date. Without fixing such
record date the Directors may for any of such purposes close the transfer books
for all or any part of such period.

      If no record date is fixed and the transfer books are not closed, the
record date for determining stockholders having the right to notice of or to
vote at a meeting of stockholders shall be at the close of business on the day
next preceding the day on which notice is given, and the record date for
determining stockholders for any other purpose shall be at the close of business
on the day on which the Board of Directors acts with respect thereto.

      5. Replacement of Certificates. In case of the alleged loss, mutilation or
destruction of a certificate of stock, a duplicate certificate may be issued in
place thereof, upon such terms and conditions as the Directors may prescribe.

      6. Issue of Capital Stock. The whole or any part of the then authorized
but unissued shares of each class of stock may be issued at any time or from
time to time by the Board of Directors without action by the stockholders.

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      7. Reacquisition of Stock. Shares of stock previously issued which have
been reacquired by the corporation, may be restored to the status of authorized
but unissued shares by vote of the Board of Directors, without amendment of the
Articles of Organization.

                                    ARTICLE V

                        Provisions Relative to Directors,
                      Officers, Stockholders and Employees

      1. Certain Contracts and Transactions. In the absence of fraud or bad
faith, no contract or transaction by this corporation shall be void, voidable or
in any way affected by reason of the fact that the contract or transaction is
(a) with one or more of its officers, Directors, stockholders or employees, (b)
with a person who is in any way interested in this corporation or (c) with a
corporation, organization or other concern in which an officer, Director,
stockholder or employee of this corporation is an officer, director,
stockholder, employee or in any way interested. The provisions of this section
shall apply notwithstanding the fact that the presence of a Director or
stockholder, with whom a contract or transaction is made or entered into or who
is an officer, director, stockholder or employee of a corporation, organization
or other concern with which a contract or transaction is made or entered into or
who is in any way interested in such contract or transaction, was necessary to
constitute a quorum at the meeting of the Directors (or any authorized committee
thereof) or stockholders at which such contract or transaction was authorized
and/or that the vote of such Director or stockholder was necessary for the
adoption of such contract or transaction, provided that if said interest was
material, it shall have been known or disclosed to the Directors or stockholders
voting at said meeting on said contract or transaction. A general notice to any
person voting on said contract or transaction that an officer, Director,
stockholder or employee has a material interest in any corporation, organization
or other concern shall be sufficient disclosure as to such officer, Director,
stockholder or employee 

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with respect to all contracts and transactions with such corporation,
organization or other concern. This section shall be subject to amendment or
repeal only by action of the stockholders.

      2. Indemnification. Each Director and officer of the corporation, and any
person who, at the request of the corporation, serves as a director or officer
of another organization shall be indemnified by the corporation against any
cost, expense (including attorneys' fees), judgment, liability and/or amount
paid in settlement reasonably incurred by or imposed upon him in connection with
any action, suit or proceeding (including any proceeding before any
administrative or legislative body or agency), to which he may be made a party
or otherwise involved or with which he shall be threatened, by reason of his
being, or related to his status as, a Director or officer of the corporation or
of any other organization, which other organization he serves or has served as
director or officer at the request of the corporation (whether or not he
continues to be an officer or Director of the corporation or such other
organization at the time such action, suit or proceeding is brought or
threatened), unless such indemnification is prohibited by the Business
Corporation Law of the Commonwealth of Massachusetts. The foregoing right of
indemnification shall be in addition to any rights to which any such person may
otherwise be entitled and shall inure to the benefit of the executors or
administrators of each such person. The corporation may pay the expenses
incurred by any such person in defending a civil or criminal action, suit or
proceeding in advance of the final disposition of such action, suit, or
proceeding, upon receipt of an undertaking by such person to repay such payment
if it is determined that such person is not entitled to indemnification
hereunder. This section shall not affect any rights to indemnification to which
corporate personnel other than Directors and officers may be entitled by
contract or otherwise under law. This section shall be subject to amendment or
repeal only by action of the stockholders.

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                                   ARTICLE VI

                            Miscellaneous Provisions

      1. Fiscal Year. Except as from time to time otherwise determined by the
Directors, the fiscal year of the corporation shall be the twelve (12) months
ending the last day of May. Following any change in the fiscal year previously
adopted, a certificate of such change, signed under the penalties of perjury by
the Clerk or an Assistant Clerk, shall be filed forthwith with the state
secretary.

     2. Seal. The seal of this corporation shall, subject to alteration by the
Directors, bear its name, the word "Massachusetts", and the year of its
incorporation.

      3. Execution of Instruments. All deeds, leases, transfers, contracts,
bonds, notes and other obligations authorized to be executed by an officer of
the corporation in its behalf shall be signed by the President or the Treasurer
except as the Directors may generally or in particular cases otherwise
determine.

      4. Voting of Securities. Except as the Directors may otherwise designate,
the President or Treasurer may waive notice of, and appoint any person or
persons to act as proxy or attorney in fact for this corporation (with or
without power of substitution) at any meeting of stockholders or shareholders of
any other corporation or organization, the securities of which may be held by
the corporation.

      5. Corporate Records. The original, or attested copies, of the Articles of
Organization, By-laws and records of all meetings of incorporators and
stockholders, and the stock and transfer records, which shall contain the names
of all stockholders and the record address and the amount of stock held by each,
shall be kept in Massachusetts at the principal office of the corporation or at
an office of its transfer agent or of the Clerk or of its resident agent. Said
copies and records need not all be kept in the same office. They shall be
available at all reasonable times to the inspection of any stockholder for any
proper purpose but not to secure a list of stockholders or other information for
the purpose of selling said list or information or 

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copies thereof or of using the same for a purpose other than in the interest of
the applicant, as a stockholder, relative to the affairs of the corporation.

      6. Articles of Organization. All references in these By-laws to the
Articles of Organization shall be deemed to refer to the Articles of
Organization of the corporation, as amended and in effect from time to time.

      7. Amendments. These By-laws, to the extent provided in these By-laws, may
be amended or repealed, in whole or in part, and new By-laws adopted either (a)
by the stockholders at any meeting of the stockholders by the affirmative vote
of the holders of at least a majority in interest of the capital stock present
and entitled to vote, provided that notice of the proposed amendment or repeal
or of the proposed making of new By-laws shall have been given in the notice of
such meeting, or (b) if so authorized by the Articles of Organization, by the
Board of Directors at any meeting of the Board by the affirmative vote of a
majority of the Directors then in office, but no amendment or repeal of a By-law
shall be voted by the Board of Directors and no new By-law shall be made by the
Board of Directors which alters the provisions of these By-laws with respect to
removal of Directors, or the election of committees by Directors and the
delegation of powers thereto, nor shall the Board of Directors make, amend or
repeal any provision of the By-laws which by law, the Articles of Organization
or the By-laws requires action by the stockholders. Not later than the time of
giving notice of the meeting of stockholders next following the making,
amending, or repealing by the Directors of any By-law, notice thereof stating
the substance of such change shall be given to all stockholders entitled to vote
on amending the By-laws. Any By-law or amendment of a By-law made the Board of
Directors may be amended or repealed by the stockholders by affirmative vote as
above provided in this Section 7.

      8. 1987 Massachusetts Control Share Acquisition Act. The 1987
Massachusetts Control Share Acquisition Act, Chapter 110D of the Massachusetts
General Laws, as it may be amended from time to time, shall not
apply to the corporation.