XIII 



NERVOUS SYSTEM 



589 



the formation of a head-fold in the region of the pro-amnion 

 (p. 583, Figs. 153, E, and 158, A) the anterior end of the 

 embryo (head-process) becomes marked off from the 

 blastoderm. 



In craniate Vertebrates the anterior end of the hollow 

 medullary tube becomes dilated, forming three bulb-like 



m.f 



nc 



ms.cf 



m.c 



m-s.s 



ect 



ccel- 



-mes' 

 -mes" 



FIG. 155. Transverse sections across the body of a chick embryo at the twenty- 

 fourth to thirtieth hour of incubation, to show stages in the formation of the 

 medullary cord and coelome. (x 100.) 



oo. aorta ; mes. mesoderm ; mes' parietal, and mes" visceral layer of mesoderm ; 

 m. f. medullary fold ; m. g. medullary groove ; ms. d. mesonephric duct ; ms. s. 

 mesodermal segment ; nc. notochord. 



veilings the fore-brain (Fig. 156, A, /. b), mid-brain 

 (m. b), and hind-brain (h. b). Soon another hollow 

 swelling grows forwards from the first vesicle (B, prs. en), 

 and the third gives off a similar hollow outgrowth (cblm) 



