FIG. 23. Diagram showing the relation of the extra- 

 embryonal membranes. A, bird or reptile, with 

 functional yolk-sac and respiratory allantois; B, 

 mammal with functionless yolk-sac and with the 

 allantois converted into an umbilical cord and 

 placenta. From Wilder' s History of the human 

 body, published by Henry Holt & Company. 



