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                                                                   EXHIBIT 10.21
                                                                                
                              THE DTM CORPORATION
                            1998 STOCK OPTION PLAN
                 (Amended and Restated Effective May 13, 1998)
               (Subject to Shareholder Approval on May 13, 1998)


     1.  PURPOSE.  The purpose of the Plan is to promote the interests of the
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shareholders by providing stock-based incentives to employees of the Company to
align their interests with shareholders and to motivate them to put forth
maximum efforts toward the continued growth, profitability and success of the
Company.  In furtherance of this objective, stock options, stock appreciation
rights, performance shares, restricted shares, common stock, and/or other
incentive awards may be granted in accordance with the provisions of this Plan
to some or all employees.

     2.  ADMINISTRATION.  The Board of Directors of the Company (the "BOARD")
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shall have authority to administer the Plan with respect to Participants who are
subject to Section 16 of the Securities Exchange Act of 1934 ("SECTION 16
INSIDERS") but may delegate such authority to the Compensation Committee (the
"COMMITTEE") of the Board.  The Committee shall administer the Plan with respect
to all other Participants.  The Committee shall consist of at least three
members who shall not be eligible to participate in the Plan.

         The Committee may delegate to the Chief Executive Officer and to other
senior officers of the Company the authority to make awards under the Plan with
respect to not more than ten percent of the shares authorized under the Plan,
pursuant to such conditions and limitations as the Committee may establish.
However, only the Board or the Committee may make awards to Section 16 Insiders.

         For purposes of the Plan, the PLAN ADMINISTRATOR shall mean the
particular entity, whether the Board, the Committee or the Chief Executive
Officer or other officer which is authorized to administer the Plan with respect
to one or more classes of eligible persons, to the extent such entity is
carrying out its administrative functions under the Plan with respect to the
persons under its jurisdiction. All decisions, action or interpretations of the
Plan Administrator shall be final, conclusive and binding on all parties.

     3.  SHARES AVAILABLE FOR THE PLAN.  An aggregate of 300,000 shares of
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common stock of the Company shall be available for delivery pursuant to the
provisions of the Plan.  Such shares may be either authorized but unissued
shares or treasury shares.  Any shares awarded under the Plan which are not
issued or otherwise are returned to the Company, whether because awards have
been forfeited, lapsed, expired, been canceled shall again be available for
other awards under the Plan.  However, upon surrender of a stock option or
exercise of any related stock appreciation right, or in the event that shares
are withheld to satisfy tax withholding obligations, the number of shares
subject to the surrendered option or withheld shall be charged against the
maximum number of shares issuable under the Plan and shall not be available for
future awards.

     4.  LIMITATION ON AWARDS.  No individual employee may receive awards under
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this Plan with respect to more than 50,000 shares in any calendar year.
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     5.  TERM.  No awards may be made under this Plan after May 13, 2003.
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     6.  ELIGIBILITY.  Awards under the Plan may be made to any full-time
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employee of the Company or any Subsidiary corporation.  Directors who are not
full-time employees are not eligible to participate.

     7.  STOCK OPTIONS. The Plan Administrator may in its discretion from time
         -------------
to time grant to eligible employees options to purchase, at a price not less
than 100% of the fair market value on the date of grant (the "option price"),
common stock of the Company, subject to the conditions set forth in this Plan.
The Plan Administrator may not reduce the option price of any stock option grant
after it is made, except in connection with a Corporate Reorganization, nor may
the Plan Administrator agree to exchange a new lower priced option for an
outstanding higher priced option.

         The Plan Administrator, at the time of granting to any employee an
option to purchase shares or any related stock appreciation right or limited
stock appreciation right under the Plan, shall fix the terms and conditions upon
which such option or appreciation right may be exercised (including a schedule
for vesting, the granting of rights to exercise after termination of Service in
addition to those specified herein, and any other terms and conditions deemed
appropriate by the Plan Administrator and not inconsistent with this Plan) and
may designate options incentive stock options pursuant to Section 422 of the
Internal Revenue Code of 1986, as amended (the "Internal Revenue Code") or any
other statutory stock option that may be permitted under the Internal Revenue
Code from time to time, provided, however that (i) the date on which such
options and related appreciation rights shall expire, if not exercised, may not
be later than ten years after the date of grant of the option, (ii) in the case
of options designated as incentive stock options (as defined in Section 422 of
the Internal Revenue Code), the aggregate fair market value of stock optioned to
an employee (determined at time of grant) under this plan or any other plan of
this Company and its subsidiaries with respect to which incentive stock options
are exercisable for the first time by such employee during any calendar year
shall be limited to $100,000 (unless such Section 422 limit is revised, then in
conformance with such revision) and (iii) in case of any other statutory stock
option permitted under the Internal Revenue Code, then in accordance with such
provisions as in effect from time to time.

         Within the foregoing limitations, the Plan Administrator shall have the
authority in its discretion to specify all other terms and conditions, including
but not limited to provisions for the exercise of options in installments, the
time limits during which options may be exercised, and in lieu of payment in
cash, the exercise in whole or in part of options by tendering common stock of
the Company owned by the employee, valued at the fair market value on the date
of exercise or other acceptable forms of consideration equal in value to the
option price. The Plan Administrator may, in its discretion, issue rules or
conditions with respect to utilization of common stock for all or part of the
option price, including limitations on the pyramiding of shares.

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    8.  STOCK APPRECIATION RIGHTS. The Plan Administrator may, in its
        -------------------------
discretion, grant stock appreciation rights and limited stock appreciation
rights (as hereinafter described) in connection with any stock option, either at
the time of grant of such stock option or any time thereafter during the term of
such stock option. Except for the terms of this Plan with respect to limited
stock appreciation rights, each stock appreciation right shall be subject to the
same terms and conditions as the related stock option and shall be exercisable
at such times and to such extent as the Plan Administrator shall determine, but
only so long as the related option is exercisable. The number of stock
appreciation rights or limited stock appreciation rights shall be reduced not
only by the number of appreciation rights exercised but also by the number of
shares purchased upon the exercise of a related option. A related stock option
shall cease to be exercisable to the extent the stock appreciation rights or
limited stock appreciation rights are exercised. Upon surrender to the Company
of the unexercised related stock option, or any portion thereof, a stock
appreciation right shall entitle the optionee to receive from the Company in
exchange therefor (a) a payment in stock as determined below, or (b) to the
extent determined by the Plan Administrator, the cash equivalent of the fair
market value of such payment in stock on the exercise date had the employee been
awarded a payment in stock instead of cash, or any combination of stock and
cash. The number of shares which shall be issued pursuant to the exercise of
stock appreciation rights shall be determined by dividing (1) the total number
of stock appreciation rights being exercised multiplied by the amount by which
the fair market value of a share of common stock of the Company on the exercise
date exceeds the option price of the related option, by (2) the fair market
value of a share of common stock of the Company on the exercise date. No
fractional shares shall be issued.

         The grant of limited stock appreciation rights will permit a grantee to
exercise such limited stock appreciation rights for cash during a sixty-day
period commencing on the date on which any of the events described in the
definition of Change of Control occurs. The amount of cash received upon the
exercise of any limited stock appreciation rights shall equal the excess, if
any, of the fair market value of a share of the Company's common stock on the
date of exercise of the limited stock appreciation rights, over the option price
of the stock option to which the limited stock appreciation rights relate.

     9.  PERFORMANCE SHARE AWARDS.  The Plan Administrator may make awards in
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common stock subject to conditions established by the Plan Administrator which
may include attainment of specific performance objectives ("Performance Share
Awards").  Performance Share Awards may include the awarding of additional
shares upon attainment of the specified performance objectives.

     10. PERFORMANCE OBJECTIVES.  Performance objectives that may be used under
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the Plan include Net Income, Pretax Income, Consolidated Operating Income,
Segment Operating Income, Return on Equity, Operating Income Return on Net
Capital Employed, Return on Assets, Cash Flow, Working Capital and Earnings per
Share of Common Stock of the Company (the "Performance Objectives").  The
Performance Objectives shall be calculated without regard to any change in
accounting standards adopted pursuant to the Financial Accounting Standards
Board after the goal for a Performance Objective is adopted which will affect
the performance measure by 10 percent or more.

     11. RESTRICTED SHARES.  The Plan Administrator may make awards in common
         -----------------                                                   
stock subject to conditions, if any, established by the Plan Administrator which
may include continued

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service with the Company or its subsidiaries. Any award of Restricted Shares
which is conditioned upon continued Service shall be conditioned upon continued
Service for a minimum period of two years and ten months following the award,
except in the case of death, disability or retirement. The maximum number of
Restricted Shares that may be awarded under the plan shall be 50,000 shares.

     12. OTHER AWARDS.  The Plan Administrator may make awards authorized under
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this Plan in Units, the value of which is based, in whole or in part, on the
value of the Company's common stock, in lieu of making such awards in common
stock.  The Plan Administrator may provide for the deferral of cash-based awards
under such terms and conditions as in its discretion it deems appropriate.

     13. FAIR MARKET VALUE.  For all purposes of this Plan, the fair market
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value of a share of stock shall be the mean of the high and low prices of the
Company's common stock on the relevant date as reported on the Nasdaq National
Market System -- based on a generally recognized report or reporting service,
or, if no sale was made on such date, then on the next preceding day on which
such a sale was made.

     14. TERMINATION OF SERVICE.  Upon the termination of Service of any 
         ----------------------                                          
employee for any reason, his or her options and any related appreciation rights
shall terminate at that time with respect to all shares which were not then
purchasable by him or her, provided, however, that if the termination of Service
is by reason of death, disability or retirement the Plan Administrator may in
its sole discretion provide that such options and related appreciation rights
shall not terminate upon death, disability or retirement and may become
immediately exercisable or continue to become exercisable in accordance with the
terms of the original grant. Terms for exercise of options for shares that are
purchasable by an employee upon termination of Service for reasons other than
death, disability or retirement shall be specified by the Plan Administrator.

         For purposes of the Plan, SERVICE shall mean the performance of
services for the Company (or any Parent or Subsidiary corporation) by a person
in the capacity of an employee, a non-employee member of the board of directors
or a consultant or independent advisor, except to the extent otherwise
specifically provided in the documents evidencing the award under the Plan.

         For purposes of the Plan, PARENT shall mean any corporation (other than
the Company) in an unbroken chain of corporations ending with the Company,
provided each corporation in the unbroken chain (other than the Company) owns,
at the time of the determination, stock possessing fifty percent (50%) or more
of the total combined voting power of all classes of stock in one of the other
corporations in such chain.

         For purposes of the Plan, SUBSIDIARY shall mean any corporation (other
than the Company) in an unbroken chain of corporations beginning with the
Company, provided each corporation (other than the last corporation) in the
unbroken chain owns, at the time of the determination, stock possessing fifty
percent (50%) or more of the total combined voting power of all classes of stock
in one of the other corporations in such chain.

     15. ASSIGNABILITY.  Options and any related appreciation rights and other
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awards granted under this Plan shall not be transferable other than by will or
the laws of descent and distribution or by such other means as the Plan
Administrator may approve from time to time.

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     16. CORPORATE REORGANIZATION.  The number and kind of shares authorized for
         ------------------------                                               
delivery under the Plan and the price at which shares may be purchased may be
adjusted appropriately in the event of any stock split, stock dividend,
combination of shares, merger, consolidation, reorganization, or other change in
the structure of the Company or the nature of the shares of the Company.  The
determination of what adjustments, if any, are appropriate shall be made in the
discretion of the Plan Administrator.

     17. PLAN ADMINISTRATOR'S DETERMINATION.  The Plan Administrator's
         ----------------------------------                           
determinations under the Plan, within the scope of its administrative functions
under the Plan, including without limitation, determinations of the employees to
receive awards or grants, the form, amount and timing of such awards or grants,
the terms and provisions of such awards or grants and the agreements evidencing
same, and the establishment of Performance Objectives need not be uniform and
may be made by it selectively among employees who receive, or are eligible to
receive awards or grants under the Plan whether or not such employees are
similarly situated.

     18. LEAVE OF ABSENCE.  The Plan Administrator shall be entitled to make
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such rules, regulations and determinations as it deems appropriate under the
Plan in respect of any leave of absence taken by an employee. Without limiting
the generality of the foregoing, the Plan Administrator shall be entitled to
determine (i) whether or not any such leave of absence shall constitute a
termination of Service within the meaning of the Plan and (ii) the impact, if
any, of any such leave of absence on awards under the Plan theretofore made to
any employee who takes such leave of absence.

     19. WITHHOLDING TAXES.  The Plan Administrator shall have the right to
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require any Federal, state or local withholding tax requirements to be satisfied
by withholding shares of common stock or other amounts which would otherwise be
payable under the Plan.

     20. RETENTION OF SHARES.  If shares of common stock are awarded subject to
         -------------------                                                   
attainment of Performance Objectives, continued service with the Company or
other conditions, the shares may be registered in the employees' names when
initially awarded, but possession of certificates for the shares shall be
retained by the Secretary of the Company for the benefit of the employees, or
shares may be registered in book entry form only, in both cases subject to the
terms of this Plan and the conditions of the particular awards.  In either
event, each employee shall have the right to receive all dividends and other
distributions made with respect to such awards registered in his or her name and
shall have the right to vote or execute proxies with respect to such registered
shares.

     21. FORFEITURE OF AWARDS.  Any awards or parts thereof made under this plan
         --------------------                                                   
which are subject to Performance Objectives or other conditions which are not
satisfied, shall be forfeited, and any shares of common stock issued shall
revert to the Treasury of the Company.

     22. CONTINUED EMPLOYMENT OR SERVICE RIGHTS.  Nothing in the Plan or in any
         --------------------------------------                                
agreement entered into pursuant to the Plan shall confer upon any employee the
right to continue in Service of the Company or affect any right which the
Company may have to terminate Service of such employee.

     23. CHANGE IN CONTROL.  For purposes of the Plan, a CHANGE IN CONTROL shall
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mean:
             (i)     The acquisition by any individual, entity or group (within
the meaning of Section 13(d)(3) or 14(d)(2) of the Securities Exchange Act of
1934, as amended (the "Exchange Act")), of beneficial ownership (within the
meaning of Rule 13d-3 promulgated under the Exchange Act) of 50% or more of
either (A) the then outstanding shares of common stock of the Company (the
"Outstanding Company Common Stock") or (B) the combined voting power of the then
outstanding voting securities of the Company entitled to vote generally in the
election of directors (the "Outstanding Company Voting Securities"); provided,
however, that the following acquisitions shall not constitute a Change of
Control: (A) any acquisition directly from the Company (other than by exercise
of a conversion privilege), (B) any acquisition by the Company or any of its
subsidiaries, (C) any acquisition by any employee benefit plan (or related
trust) sponsored or maintained by the Company or any of its subsidiaries or (D)
any acquisition by any corporation with respect to which, following such
acquisition, more than 70% of, respectively, the then outstanding voting
securities of such corporation entitled to vote generally in the election of
directors is then beneficially owned, directly or indirectly, by all or
substantially all of the individuals and entities who were the beneficial
owners, respectively, of the Outstanding Company Common Stock and Company Voting
Securities immediately prior to such acquisition in substantially the same
proportions as their ownership, immediately prior to such acquisition, of the
Outstanding Company Common Stock and Outstanding Company Voting Securities, as
the case may be; or

             (ii)   During any period of two consecutive years, individuals who,
as of the beginning of such period, constitute the Board (the "Incumbent
Board"), cease for any reason to constitute at least a majority of the Board;
provided, however, that any individual becoming a director subsequent to the
beginning of such period whose election, or nomination for election by the
Company's shareholders, was approved by a vote of at least a majority of the
directors then comprising the Incumbent Board shall be considered as though such
individual were a member of the Incumbent Board, but excluding, for this
purpose, any such individual whose initial assumption of office occurs as a
result of either an actual or threatened election contest (as such terms are
used in Rule 14a-11 of Regulation 14A promulgated under the Exchange Act); or

             (iii)  approval by the shareholders of the Company of a
reorganization, merger or consolidation, in each case, with respect to which all
or substantially all of the individuals and entities who were the beneficial
owners, respectively, of the Outstanding Company Common Stock and Outstanding
Company Voting Securities immediately prior to such reorganization, merger or
consolidation, do not, following such reorganization, merger or consolidation,
beneficially own, directly or indirectly, more than 70% of, respectively, the
then outstanding shares of common stock and the combined voting power of the
then outstanding voting securities entitled to vote generally in the election of
directors, as the case may be, of the corporation resulting from such
reorganization, merger or consolidation in substantially the same proportions as
their ownership, immediately prior to such reorganization, merger or
consolidation of the Outstanding Company Common Stock and Outstanding Company
Voting Securities, as the case may be; or

             (iv)   Approval by the shareholders of the Company of (A) a
complete liquidation or dissolution of the Company or (B) a sale or other
disposition of all or substantially all of the assets of the Company, other than
to a corporation, with respect to which following such sale or other
disposition, more than 70% of, respectively, the then outstanding shares of
common stock of such

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<PAGE>
 
corporation and the combined voting power of the then outstanding voting
securities of such corporation to vote generally in the election of directors is
then beneficially owned, directly or indirectly, by all or substantially all of
the individuals and entities who were the beneficial owners, respectively, of
the Outstanding Company Common Stock and Outstanding Company Voting Securities
immediately prior to such sale or other disposition in substantially the same
proportion as their ownership, immediately prior to such sale or other
disposition, of the Outstanding Company Common Stock and Outstanding Company
Voting Securities, as the case may be.

     24. EFFECT OF CHANGE IN CONTROL.  In the event of any Change in Control in
         ---------------------------                                           
connection with which outstanding options and any related appreciation rights
are not continued pursuant to the following provisions, such options and related
rights shall automatically accelerate so that each such option shall,
immediately prior to the effective date of the Change in Control, become fully
exercisable with respect to the total number of shares at the time subject to
such option and may be exercised for any or all of those shares as fully-vested
shares of the Company's common stock.  Holders of options and any related
appreciation rights shall be given reasonable advance notice of the effective
date of the Change of Control and a reasonable opportunity to deliver notice of
exercise in a timely manner.

         Outstanding options and any related appreciation rights may be
continued and shall not so accelerate in connection with a Change in Control if
and to the extent: (i) such options are to be assumed by the successor
corporation (or parent thereof), to be replaced with a comparable option to
purchase shares of the capital stock of the successor corporation (or parent
thereof) or to otherwise continue in effect, in all material respects, pursuant
to the provisions of the Change in Control, or (ii) such options are to be
replaced with a cash or other equivalent incentive program of the successor
corporation which preserves the spread existing at the time of the Change in
Control on the option shares for which the options are not otherwise at that
time exercisable (the excess of the fair market value of those shares over the
aggregate option exercise price payable for those shares) and provides for
subsequent payout in accordance with the same exercise/vesting schedule
applicable to those option shares. The determination of option comparability or
equivalence under clause (i) and (ii) above shall be made by the Plan
Administrator.

         Notwithstanding the foregoing, options shall not accelerate or be
continued if such acceleration or continuation contravenes or conflicts with
terms or conditions established for such options at the time of the option
grant.

         Immediately following the consummation of the Change in Control, all
outstanding options and related stock appreciation rights shall terminate and
cease to be outstanding, except to the extent continued pursuant to the
foregoing provisions or to the extent necessary to allow for the completion of
exercise transactions for which notice of exercise was timely given in
connection with the Change in Control.

         Each option which is assumed in connection with a Change in Control
shall be appropriately adjusted, immediately after such Change in Control, to
apply to the number and class of securities which would have been issuable to
the employee in consummation of such Change in Control had the option been
exercised immediately prior to such Change in Control. Appropriate adjustments
to reflect such Change in Control shall also be made to (i) the exercise price
payable per share under

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<PAGE>
 
each outstanding option, provided the aggregate exercise price payable for such
securities shall remain the same, (ii) the maximum number and/or class of
securities available for issuance over the remaining term of the Plan, (iii) the
maximum number and/or class of securities for which any one person may be
granted securities under the Plan per calendar year, and (iv) the maximum number
of Restricted Shares that may be awarded under the Plan.

         The Plan Administrator shall have full power and authority, exercisable
either at the time the option is granted or at any time while the option remains
outstanding, to provide for the automatic acceleration of one or more
outstanding options under the Plan in the event the employee's Service
subsequently terminates by reason of an Involuntary Termination within a
designated period (not to exceed eighteen (18) months) following the effective
date of any Change in Control in which those options or other awards are assumed
or replaced or otherwise continued in effect and do not otherwise accelerate.
Any options so accelerated shall remain exercisable for fully-vested shares
until the earlier of (i) the expiration of the option term or (ii) the
expiration of the one (1)-year period measured from the effective date of the
Involuntary Termination.

         For purposes of the Plan, INVOLUNTARY TERMINATION shall mean the
termination of the Service of any employee which occurs by reason of:


               a.  such employee's involuntary dismissal or discharge by the
     Company (or successor corporation) for reasons other than Misconduct, or


               b.  an employee's voluntary resignation due to a change in
     employee's position with the Company (or Parent or Subsidiary employing
     such individual) or successor corporation which materially reduces the
     employee's duties and responsibilities or a reduction in the employee's
     level of cash compensation (including base salary, fringe benefits and
     target bonus under any corporate performance-based bonus or incentive
     programs) by more than fifteen percent (15%), in the aggregate, provided
     and only if such change or reduction is effected without the employee's
     consent.

               c.  such other material change or changes in such employee's
     employment as are determined appropriate by the Plan Administrator at the
     time of the option grant.

         For purposes of the Plan, MISCONDUCT shall mean the commission of any
act of fraud, embezzlement or dishonesty by an employee or other individual
receiving an award under the Plan, any unauthorized use or disclosure by such
person of confidential information or trade secrets of the Company (or any
Parent or Subsidiary) or successor corporation, or any other reckless conduct or
knowing or intentional misconduct by such person that materially adversely
affects (or that could be predicted to materially adversely affect in the
future) the performance of the job functions of such person, in his or her
position with the Company (or any Parent, Subsidiary or successor). The
foregoing definition shall not be deemed to be inclusive of all the acts or
omissions which the Company (or any Parent or Subsidiary) or successor
corporation may consider as grounds for the dismissal or discharge of any
employee or other person in the Service of the Company (or any Parent or
Subsidiary) or successor corporation.

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<PAGE>
 
         The Plan Administrator may, either at the time of approving Performance
Share Awards, awards of Restricted Shares or other awards under the Plan, or at
any time such awards remain outstanding, apply the provisions described above
with respect a Change In Control to such awards as it deems appropriate in its
discretion.

     25. COMPLIANCE WITH LAWS AND REGULATIONS.  Notwithstanding any other
         ------------------------------------                            
provisions of the Plan, the issuance or delivery of any shares may be postponed
for such period as may be required to comply with any applicable requirements of
any national securities exchange or any requirements under any other law or
regulation applicable to the issuance or delivery of such shares, and the
Company shall not be obligated to issue or deliver any such shares if the
issuance or delivery thereof shall constitute a violation of any provision of
any law or any regulation of any governmental authority, whether foreign or
domestic, or any national securities exchange.

     26. AMENDMENT.  The Board may alter or amend the Plan, in whole or in part,
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from time to time, or terminate the Plan at any time, provided however, that no
amendment shall be made without the approval of the shareholders if required
pursuant to applicable laws or regulations, and no such action shall adversely
affect any rights or obligations with respect to awards previously made under
the Plan, unless the holder of such awards consents in writing.

                                       9