xm 



XERVOi PEM 



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 



coeI---~ 



nc 



end 



ms.d 



m.c 



ms s 



ect 



ccel-- 



enaf 



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. ( .. 100.) 



ao. aorta ; mes. mesoderm ; mes" parietal, and mesi" visceral layer of me-oderm ; 

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

 mesodermal segment ; nc. notochord. 



swellings 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) 



