Mass. Gen. Laws ch. 90, § 22H

No person shall transport an animal in the back of a motor vehicle in a space intended for a load on the vehicle on a public way unless such space is enclosed or has side and tail racks to a height of at least 46 inches extending vertically from the floor, the animal is cross tethered to the vehicle, the animal is protected by a secured container or cage or the animal is otherwise protected in a manner which will prevent the animal from being thrown or from falling or jumping from the vehicle. Whoever violates the provisions of this section shall be punished by a fine of not less than $50.