584 NERVOUS SYSTEM CHAP, xn 



endoderm. Thus the coelome is formed not as an entero- 

 coele, but as a schizoccde. 



In Vertebrates each mesoderm-band becomes differ- 

 entiated into a dorsal portion, the vertebral plate, which 

 soon loses its ccelomic space, and a ventral portion, the 

 lateral plate, which is divided into parietal and visceral 

 layers by the ccelome (Figs 153 D, 157 and 158). The 

 vertebral plate undergoes metameric segmentation, becom- 

 ing divided into a row of squarish masses, the mesodermal 

 segments or " protovertebra " (pr. v\ from which the muscular 

 segments or myomeres ^e formed (p. 203), and also the 

 vertebral column, the segmentation of which alternates with 

 that of the myomeres. 



Outline of the development of the chief organs in the 

 Craniata (compare pp. 201-210). The nervous system, 

 as well as the essential parts of the sensory organs are, as 

 we have seen, in all cases formed from the ectoderm (pp. 

 202, 209, and Figs. 64, 65, and 153), and in craniate Verte- 

 brates the anterior end of the hollow medullary tube becomes 

 dilated, forming three bulb-like swellings the fore-brain (Fig. 

 T 59> A >/ ^)> mid-brain (m. b), and hind-brain (h. b). Soon 

 another hollow swelling grows forwards from the first vesicle 

 (v,prs. en), and the third gives off a similar hollow out- 

 growth (cblni) from its dorsal surface. The brain now con- 

 sists of five divisions : the prosencephalon (prs. en) and 

 diencephalon (dien) derived from the fore-brain, with the 

 pineal structures (pn. b, pn. e) and the infundibulum 

 and pituitary body (inf. pty] ; the mid-brain or mesen- 

 cephalon (m. b) which gives rise to the optic lobes and crura 

 cerebri ; and the epencephalon or cerebellum (cblni) and 

 metencephalon or medulla oblongata (med. obl\ derived from 

 the hind-brain. The original cavity of the brain becomes 



