STRUCTURE OF THE VERTEBRATES 



93 



outpocketing of the hind gut and grows outward toward the 

 chorion, lying between the amnion and yolk sac. 



Up to this time the more fundamental processes of the pla- 

 cental have parallelled those of the reptile; but the later develop- 

 ment of nutritive processes differs materially from those of other 



Embryonic knot 



Neural groove 



Placenta 



Allantois 



Yolksac 



Early Embryo 



Umhilical cord 

 Later Embryo 



\terine ivall 



Placental Attachment 



Tig. 40. Embrj^ology of the Mammal. Diagrams D to H inclusive are 

 cross sections to show the development of ectoderm, endoderm and 3'olk 

 sac from the embryonic knot of cells. Drawing I and J are longitudinal 

 sections to show the development of the embryo, the umbilical cord, 

 and placenta. Drawing K shows the attachment of the embryonic placenta 

 to the maternal uterine wall, and the placental villi extending into the 

 blood sinus of the uterus. 



