MULTICELLULAR ANIMALS 



217 



the development of the vertebrate embryo by the sinking inward of 

 a plate of ectoderm along the dorsal mid-line of the newly estab- 

 lished body. This plate soon becomes a distinct groove, the margins 

 of which approach each other and finally fuse together, thus form- 

 ing a tube of ectoderm embedded in the surrounding mesoderm 

 (Fig. 147). So the neural tube is established early in the life of the 

 individual; by differentiation of various regions of this tube the 

 central nervous system is set up. Dorsal and lateral to the primitive 



PROSEN- 

 CEPHALON 



MESENCEPHALON 



PHOMBEN- 

 CEPHALON 



MESENCEPHALON 



DIENCEPHALON 



METENCEPHALON 



TELENCEPHALON 



OPTIC VESICLE 



MYELENCEPHALON 



B. 



Fig. 147. — Two early stages in the embryonic development of a vertebrate brain. 

 In A the view is dorsal and the brain consists essentially of three parts. In B the 

 view is lateral and the differentiation of five parts is clear. 



neural tube there remain masses of ectodermal cells that are not 

 incorporated into the tubular central nervous system but are des- 

 tined to form ganglia associated with the neural tube and the 

 ganglia of the sympathetic system. These masses of potentially 

 nervous ectodermal cells are spoken of as the neural crest. 



The Central Nervous System. Essentially then, the cen- 

 tral nervous system of the vertebrates is a tube situated dorsal to the 

 coelom, the walls of which are composed of nerve cells and associ- 

 ated connective tissues. During development in all vertebrates the 

 anterior region of the tube, by unequal growth, dilates to form three 

 primary expansions, the fore-, mid-, and hind-brains, or, more 



