Mass. Gen. Laws ch. 158, § 29: Assessment upon shares

Section 29. Every corporation may, from time to time at a legal meeting called for the purpose, assess upon each share such amount of money as it considers proper, not exceeding in the whole the par value thereof, unless the price is fixed as provided by section eighteen of chapter one hundred and sixty-four or section seven of chapter one hundred and sixty-six, and, in such case, not exceeding said price. Such assessment shall be paid to the treasurer at such times and by such instalments as the corporation directs.