DEVELOPMENT 



273 



nion and has served temporarily as a respiratory membrane. The 

 development of these two embryonic membranes, to meet new 

 conditions of embryonic existence, makes possible not only a 

 more rapid and sure interchange of materials between the embryo 

 and its surroundings, but also affords greater protection for the 

 growth of complicated structures. (Fig. 176.) 



ABC 



Fig. 177A. - Photographs of early stages in the development of the egg 

 of the Rabbit. A, two-cell stage, 24 hours after fertilization, in thick surround- 

 ing membrane (zona pellucida); B, four-cell stage, 29 hours; C, eight-cell 

 stage, 32 hours. Highly magnified. (After Streeter.) 



Finally, the eggs of typical Mammals, including Man, though 

 not provided with so large an amount of yolk because food is 



A B 



Fig. 177B. — Human embryos. A, one month old (6.7 mm.), showing arm 

 and leg buds, caudal end, umbilical cord, heart, gill slits, olfactory pit, and 

 eye; B, six weeks old (19 mm.), showing developing hands and feet, elbow and 

 knee, nose, eye, and ear. See Fig. 235. (After Streeter.) 



supplied to the developing embryo by the blood vascular system 

 of the mother, nevertheless inherit from lower forms the embryonic 



