34 HISTORY OF THE HUMAN BODY 



in an immature state, but are unprotected by an egg-shell, and 

 are matured in an external abdominal pouch (marsupium) until 

 able to care for themselves. The relation between the mono- 

 tremes and the modern marsupials is hardly close enough to 

 justify an immediate succession, but suggests that each group, 

 as we now know it, has descended from more primitive an- 

 cestors that were thus related; that is, that the ancestor of 

 modern marsupials was a direct descendant of the ancestor of 

 the monotremes. 



Beyond the marsupials all the mammals are placenta!, that 

 is, the embryos are retained for a longer time within the 

 uterus of the parent and are nourished by means of an organ 

 formed in part from the mucous membrane of the uterus and 

 in part from tissue furnished by the embryo but not included 

 within its body. This organ is termed the placenta and is 

 connected with the body of the embryo through an umbilical 

 cord. This cord contains fetal blood vessels which connect 

 proximally with the main circulatory system of the embryo 

 and develop distally into a system of capillaries that lie in villi 

 in the embryonal portion of the placenta, obtaining their 

 nourishment and effecting the interchange of respiratory gases 

 through osmotic transmission. There is thus no direct organic 

 continuity between mother and offspring, and neither nerves 

 nor blood vessels are continuous from one to the other. In- 

 deed, in the lower placental mammals the connection between 

 the maternal and embryonal portions of the placenta is very 

 loose and the two easily separate at birth, although in the 

 higher forms the connection becomes more intimate and the 

 separation takes place between the muscular and mucous coat 

 of the uterus, thus involving an actual loss of maternal 

 tissue. 



The placental mammals, although their appearance was com- 

 paratively recent, geologically speaking, have specialized in all 

 directions, and now occupy almost every available environment, 

 not only of the land, but of the water. Some are fitted to 

 pursue and drag down large herbivorous animals, while others 

 feast upon dead bodies or suck the blood of the living after 



