1 82 SECTIONS DURING THE SECOND AND THIRD DAY. 



In the third section (fig. 117) the body walls have become 

 nearly vertical, the folding of the splanchnopleure is nearly 

 completed, and it is only for a small region that the alimentary 

 tract is open, by the vitelline duct, to the yolk-sack. 



These three sections further illustrate (i) the gradual diffe- 



ffJ3 



FIG. 118. EMBRYO CHICK AT THE END OF THE FOURTH DAY SEEN AS A 

 TRANSPARENT OBJECT. 



The amnion has been completely removed, the cut end of the somatic stalk is 

 shewn at S.S. with the allantois (Al) protruding from it. 



C.H. cerebral hemisphere; F.B. vesicle of the third ventricle with the pineal 

 gland (Pn) projecting from its summit; M.B. mid-brain; Cb. cerebellum. IV. V. 

 fourth ventricle; Z. lens; ch.s. choroid slit. Owing to the growth of the optic cup 

 the two layers of which it is composed cannot any longer be seen from the surface, 

 but the retinal surface of the layer alone is visible. Cen. V. auditory vesicle; s.rn. 

 superior maxillary process; i f, if, etc. first, second, third and fourth visceral 

 arches; V. fifth nerve sending one branch to the eye, the ophthalmic branch, and 

 another to the first visceral arch ; VII. seventh nerve passing to the second visceral arch ; 

 G.Ph. glossopharyngeal nerve passing towards the third visceral arch ; Pg. pneumo- 

 gastric nerve passing towards the fourth visceral arch; iv. investing mass. No 

 attempt has been made in the figure to indicate the position of the dorsal wall of the 

 throat, which cannot be easily made out in the living embryo; ch. notochord. The 

 front end of this cannot be seen in the living embryo. It does not end however as 

 shewn in the figure, but takes a sudden bend downwards and then terminates in a 

 point. Ht. heart seen through the walls of the chest; M.P. muscle-plates. W. wing; 

 //. /.. hind limb. Beneath the hind limb is seen the curved tail. 



