AVES. 



151 



G.Pk 



Vff 



IV. V 



MJ3 



ss 



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; L. 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. audi- 

 tory vesicle; s.m. superior maxillary process; IF, 2 F. 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. glossopharyugeal nerve passing towards the third visceral 

 arch ; P(). pneumogastric nerve passing towards the fourth visceral arch; h\ 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. uoto- 

 chord. 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 termi- 

 nates in a point. Ht. heart seen through the walls of the chest ; M.P. muscle-plates. 

 W. wing; H.L. hind limb. Beneath the hind limb is seen the curved tail. 



siderable space between the notochord and the hypoblast, which 

 forms the rudiment of the mesentery. 



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 (1) the gradual differentiation 

 of the mesoblastic somites (fig. 115 P.v) into (a) the muscle-plates (figs. 

 116 ms and 117 nip), and (b) the tissue to form the vertebral bodies 

 and adjacent connective tissue ; (2) the formation of a mass of tissue 



