9 6 



ANIMAL BIOLOGY. 



[Part I. 



than the peripheral, while furrows beneath and parallel 



the surface separate the superficial from the deeper cells. Thus 



at the close of segmentation (i.e. before incubation) we have a 



FIG. 34. AT THE CLOSE OF SEGMENTATION. 



i. In frog. ii. In rabbit, iii. In fowl. 

 In. Inner cells. Ou. Outer cells, v. &. Pseudo-blastopore. 



k. Yolk. 



superficial layer of small columnar cells forming a distinct mem- 

 brane, and a deep layer of larger cells irregularly disposed, but 

 deeper around the edges of the disc than at its centre (Fig. 34, 

 iii.). 



The Egg Membranes. The frog's egg is closely invested by u 

 delicate vitelline membrane. External to this is the albuminous 

 envelope secreted by the walls of the oviduct. 



The fowl's egg is also closely invested by a vitelline membrane. 

 In passing down the oviduct (after impregnation) it receives a 

 thick albuminous coating of white-of-egg. At either end of the 

 egg this assumes a denser condition, and forms a twisted knotted 

 cord (chalaza). A two-layered shell-membrane, and finally 5 ' the 

 shell itself, are added in the lower parts of the oviduct. 



The rabbit's egg is surrounded by a radially striated membrane 

 (zona mdiata\ which probably does not' answer to the vitelline 

 membrane of the other types, though this membrane may exist 



