Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 158, § 28

The shares of employees' stock shall not be sold or transferred except to an employee of such corporation or to the corporation itself.  A corporation which issues employees' stock may prescribe by its by-laws the number of shares which may be held by any one employee and the method of transfer and redemption of such stock as is held by any person after he ceases to be an employee of the corporation.