Section 56. The part of the unorganized militia so drafted or accepted shall immediately be mustered under the orders of the commander-in-chief into the service of the commonwealth for three years, or for such less period as the commander-in-chief may direct, and shall be organized into new units, or assigned to organizations of the organized militia then existing. Such new organizations shall be officered, equipped, trained and governed according to the laws for the government of the organized militia. The commander-in-chief may detail officers to train and command such new organizations until their officers shall have qualified. Every member of the unorganized militia who volunteers or is drafted, who does not appear to be mustered in as required by the orders of the commander-in-chief, and does not produce a sworn certificate from a physician in good standing of physical disability so to appear, shall be prima facie a deserter and upon conviction thereof shall be punished as a court-martial may direct.