Mass. Gen. Laws ch. 156, § 18: Payment of subscriptions

Section 18. In the case of capital stock authorized to be issued for cash, whether or not to be paid in full before issue, the directors may require payment in such proportions and at such times and places as they deem proper, by making demand therefor according to the by-laws, or, in default of such by-law, by notice mailed to each subscriber or stockholder at least seven days before his subscription or any portion thereof, or any instalment due upon stock already issued, is payable.