418 



METAZOAN PHYLA 



Polar 

 body 



Zona 

 'pellucida 



Trophoderm 



E 

 Chorion 



Blasfocoel 

 (yolk sac) 



Amniotic ca\/ify 

 Mesoderm 



Amniotic 

 cavity 



Ectoderm 

 Embryo 



Entoderm Entoderm 



Placenta— ^^^^^^^ 



Extra-embry- u 

 onic coelom " 



Doay 



StCffk 



Lasttrace 

 of extra- em 

 bryonic coelom 



PIcfcentcrl 

 villi 



Neuroft 

 groove 



Fig. 287. — Diagrammatic representation of the stages in mammalian development. 

 A, egg cell, in section. B, two-celled stage. C, morula. Z), section of morula. E, 

 blastula, in section. F , development of entoderm. G, formation of amniotic cavity, and 

 separation of embryonic disc into entoderm and ectoderm. H, development of mesoderm 

 and extra-embryonic coelom. 7, formation of body stalk and beginning of placenta. 

 J , cross section of same stage as I. A', embryo in amniotic cavity, the increase of size of 

 which is about to oVjlitcrate the extra-embryonic coelom. In H to K, ectoderm is white, 

 entoderm crosslined, and mesoderm stippled. 



