266 



EMBRYOLOGY 



EGG 



MEMBRANE 



CYTOPLASM 



G 



INNER CELL MASS 



BLASTOCYST 



Fig. 170. The egg, first cleav- 

 age, and the probable structure of 

 the blastocyst in human develop- 

 ment. Above: The egg after ovula- 

 tion is surrounded by a layer of 

 cells, the corona radiata, and by a 

 transparent noncellular layer, the 

 zona pellucida. The egg has some 

 yolk granules in the cytoplasm. 

 Upper right: A two-cell stage of a 



human egg after artificial fertiliza- trophoblast 



tion. Right: Blastocyst of the monkey. The large cavity is sometimes called a 

 blastocoel but probably the term blastocyst cavity is more accurate. The embryo, 

 amnion, and yolk sac develop from the inner cell mass while the embryonic 

 portion of the placenta arises from the trophoblast. 



large amount of yolk were present. A yolk sac forms, but it is nonfunctional. 

 Moreover, a primitive streak develops, just as in the hen's egg. 



Early development 



Although in the earlier stages cleavage in the mammalian egg resembles 

 that of the sea urchin egg, the blastula formed is quite different. During 

 cleavage of the monkey egg a group of cells destined to become the embryo 

 is partially segregated from the rest of the cells, forming the inner cell mass 

 (Fig. 170) . The remaining cells form a large blastula-like cyst. In the human, 

 the walls of this cyst grow into the uterine wall to form part of the placenta. 

 Thus there is an early segregation between the embryonic tissues and the 

 extraembryonic tissues. Not all the cells of the inner cell mass develop into 

 the embryo. The amnion and the extraembryonic mesoderm also develop 

 from this mass. 



