Vertebrate Embryology 



the yolk so situated that the top of the neck 

 of the flask lies under the blastoderm. 



Although of so large a size, the yolk of the 

 hen's egg is a single cell, its great size being 

 chiefly due to the large number of yolk gran- 



shm 



alb 



alb 



FIG. 33. SEMI-DIAGRAMMATIC VIEW OF THE EGG OF THE DO- 

 MESTIC FOWL, AT THE TIME OF LAYING. (After Parker and Hes- 

 well, slightly altered from Marshall.) 



rt, air-space. /, dense layer of albumen. alb\ more fluid albumen. 

 /, blastoderm, ch^ chalaza. sh, shell, s h.m, shell-membrane. sh.m.\^ 

 sk.m.'Z, its two layers separated to enclose air-cavity. 



ules which it contains. These yolk granules 

 serve as food for the developing embryo ; and 

 it is to the abundance or scarcity of this food 

 yolk that the great variation in the sizes of 

 ova is largely due. In mammals, for example, 

 there is practically no food yolk, and the eggs 



