MAMMALIA 



411 



hours after birth, are able to walk or even to run, and require little 

 parental care except in connection with mammary feeding. In other 

 species, as in the carnivores and rodents, the young are born naked, 



FIG. 217. Diagram illustrating the formation of the umbilical cord and the 

 relations of the allantois and yolk-sac in human embryo. The heavy black line 

 represents the embryonic ectoderm; the dotted line marks the line of transition of 

 the body (embryonic) ectoderm and that of the amnion. Stippled areas, meso- 

 derm. Ac, Amniotic cavity; Al, allantoic cavity; Al, allantois; Be, exoccelom; 

 Bs, body stalk; Ch, chorion; P, placenta; Uc, umbilical cord; V, chorionic (tro- 

 phodermic) villi; Fs, yolk-sac. (From Kellicott, after McMurrich.) 



blind, and helpless and need much care for a considerable period. 

 The human infant, while not as immature as some of those just 

 mentioned, is decidedly helpless and needs care longer than any 

 other creature. 



