COMPARATIVE ZOOLOGY 



FIG. 366. The hen's egg 

 at the end of the ninth 



up in order to show its 

 shape more clearly : 

 an, inner or true am- 

 nion ; hm, hyomandibu- 



hinder portion of the alimentary canal an outgrowth is 

 formed which extends beyond the wall of the embryo 

 ta hm proper into the cavity of the amnion, 



and spreads out over the whole inner 

 surface of the shell, so that it partly 

 surrounds both embryo and inner 

 layer of the amnion (amnion proper). 

 This is the .allantois. It is full of 

 blood vessels, and it serves as the 

 embryo naturally lies with res piratorv organ until the chick 



its left side on the yolk J & 



sac, but has been lifted picks the shell and breathes by its 

 lungs. 166 The chorion is the outer- 

 most part of the allantois, and the 



iar cleft; sv, air chamber; placenta of mammals is the shaggy, 



ta, allantois; -wa, white 



or albumen ; ys, yolk VaSCular edge of the chOROn. 



The alimentary canal is at first a 



straight tube closed at both ends, the middle being 

 connected with the yolk sac. As it grows faster than 

 the body, it is thrown into a spiral coil ; and at several 

 points it dilates, to form the crop, stomach, gizzard, etc. 

 The mouth is developed from an infolding of the skin. 

 The liver is an outgrowth from 

 the digestive tube, at first a clus- 

 ter of cells, then of follicles, and 

 finally a true gland. The lungs 

 are developed on the third day 

 as a minute bud from the upper 

 part of the alimentary canal, or 

 pharynx. As they grow in size, FlG - 367- - view of embryo with 



1 its foetal membrane : am, am- 



they paSS from a SmOOth tO a nion proper ; d, dwindled yolk 



, , | , . . sac ; al, allantois ; al*, subzonal 



Cellular Condition. membrane; z, villi ; outside the 



The skeleton at the beginning S^^nStok^h^ 

 consists, like the notochord^ of a blast, z'. 

 cellular material, which gradually turns to cartilage. 

 Then minute canals containing blood vessels arise, and 



