444 



METAZOAN PHYLA 



Polar 

 body 



Zona 

 'pellucida 



Trophoderm 



^^^ Blastocoel 

 P (yolk sac) 



Amniotic 

 cavity 



Chorion 



Entoderm 



Placenta—^ 

 Amniotic cavity 



fyfesoderm 



'Aflantois 



Ectoderm- 

 Embryo 



Entoderm G 



Extra -embry- u 

 onic coelom " 



Body 

 stalk 



Last trace 

 of extra- em- 

 bryonic coelom 



Placental 

 villi 



t/eura/ 

 groove 



Fig. 343. — Diagrammatic representation of the stages in mammalian development. 

 A, egg cell, in section. B, two-celled stage. C, morula. D, 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, i/, development of mesoderm 

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

 /, cross section of same stage as I. K, embryo in amniotic cavity, the increase of size of 

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

 entoderm crosslined, and noiesoderm stippled. 



