314 Vertebrate Embryology 



At an early period, in most mammals, the 

 entoderm and mesoderm extend entirely around 

 the inside of the blastodermic vesicle, beneath 

 the subzonal layer or ectoderm. The cleavage 

 of this masoderm forms the ccelom, which ex- 

 tends entirely around the vesicle, except at 

 the body-stalk, as the extra-embryonic ccelom 

 (Fig. 93, Coe). This extra-embryonic ccelom 

 separates the ectoblast and somatic mesoblast, 

 now called the chorion, from the entoblast and 

 splanchnic mesoblast which surrounds the orig- 

 inal cavity of the blastodermic vesicle. This 

 cavity, connected, as is seen in Figures 93 and 

 94, by a wide stalk with the digestive cavity of 

 the embryo, is called the yolk-sac, though it 

 contains no yolk. The human yolk-sac is very 

 small, at its greatest development being only 

 8-10 mm. in diameter. At its earliest known 

 stage it is covered with blood vessels. As 

 development proceeds it becomes constricted 

 off from the intestine until it is connected with 

 it by merely a slender, hollow neck, the whole 

 structure being pear-shaped (Fig. 102). The 

 sac continues to increase in size until about 

 the end of the fourth week. In later stages of 

 development it is seen as a small, pear-shaped 

 mass connected with the embryo by a long slen- 



