engraving. In the lower figure a little point, P, projects downward from the front vesicle, A. As the growth of the brain proceeds, this process turns up and then over backward in the direction of the arrows in the upper figure. It goes on expanding until it forms the cerebrum, the larger mass of the brain. Although it thus turns backward, yet the line of the vital forces from the spinalis is forward and upward. The back vesicle, B, sends out a process which becomes the cerebellum or little brain, marked Cer. 1. m, in the upper figure. In the fully developed brain the three primary vesicles become central or subordinate parts.
From the first phase to the close of fetal life the brain presents a constant increase in its complexity of structure. At different parts of this period the brain resembles, in succession, those of an ascending series of the lower animals. But the brains of these lower animals are arrested, some at a lower, some at a higher point; that of man alone passes onward to completion.
In all the vertebrates, the highest division of the animal kingdom, the first part to attain a definite structure is the brain and spinal cord. On a previous page this is shown as the "Primitive Trace." And in the whole scale of life, the rank of each species of animal is determined by its development of the nervous system and of the muscles which are the direct instruments through which the brain and nerves must express themselves.