70 HISTORY OF THE HUMAN BODY 



FIG. 17. Diagrams of Amniotes, showing the relation of the extra- 

 embryonal membranes. 



(A) Sauropsidan, with . functional yolk-sac and respiratory allantois. (B) Mam- 

 mal, with functionless yolk-sac and with the allantois converted into an umbilical 

 cord and placenta. 



