As soon as the structures forming the bony boundaries of the spinal column and cranium are ready for their reception, the spinal cord and the brain are formed, constituting the cerebro-spinal system: the eyes, organs of hearing and taste, are gradually developed by the ordinary processes of cell-formation.

The alimentary canal has already been referred to in connection with the mucous layer of the blastodermic membrane, and it may be observed that from the same source the other organs of the abdominal cavity, and also the organs of respiration, are formed, and the foetal structures are, so far as general outline is concerned, complete. The subsequent processes are those of growth due to the continually added supplies of nutriment, until the young animal is fit for "separate life ", when some mysterious stimulus acts upon the uterus and causes expulsion of the foetus in the act of parturition.