272 



STRUCTURE OF THE VERTEBRATES 



separates from the covering ectoderm, small metameric masses 

 of ectodermal tissue develop on either side of the tube along 

 its dorsal periphery. These are the neural crests which give rise 

 to the sensory cells of the metameric spinal nerves. This is the 

 developmental beginning of the division of the nervous system 

 into (1) the central nervous system composed of the hrain and 

 spinal cord; and (2) the peripheral nervous system. The latter 

 includes the nerves and ganglia which develop from the central 

 system and connect it with the organs of the body. 



Fig. A 



Fig. B 



Fig. 153. Development of the Brain (Chick). 



The brain and spinal cord are continuous, structurally and 

 functionally, and the separation of the two is entirely arbitrary 

 (the foramen magnum being accepted as the dividing line). The 

 brain grows rapidly, two constrictions dividing it into three 

 major regions: (1) the forebrain (prosencephalon); (2) the 

 midbrain (mesencephalon) ; and (3) the hindbrain (rhomben- 

 cephalon). From the forebrain arise the olfactory lobes, the cere- 

 bral hemispheres, and the diencephalon. The midbrain develops 

 into the optic lobes; and the hindbrain gives rise to the cere- 

 bellum and medulla. The individual regions are discussed in 

 later paragraphs. 



