68 THE KIDDLE OF THE UNIVERSE. 



man as in all the other amniotes ; man is a true 

 amniote. 



The nourishment of the foetus in the maternal 

 womb is effected, as is well known, by a peculiar 

 organ, richly supplied with blood at its surface, called 

 the placenta. This important nutritive organ is a 

 spongy, round disc, from six to eight inches in 

 diameter, about an inch thick, and one or two pounds 

 in weight ; it is separated after the birth of the child, 

 and issues as the "after-birth." The placenta con- 

 sists of two very different parts, the foetal and the 

 maternal part. The latter contains highly-developed 

 sinuses, which retain the blood conveyed to them by 

 the arteries of the mother. On the other hand, the 

 foetal placenta is formed by innumerable branching 

 tufts or villi, which grow out of the outer surface of 

 the allantois, and derive their blood from the umbilical 

 vessels. The hollow, blood-filled villi of the foetal 

 placenta protrude into the sinuses of the maternal 

 placenta, and the slender membrane between the two 

 is so attenuated that it offers no impediment to the 

 direct interchange of material through the nutritive 

 blood-stream (by osmosis). 



In the older and lower groups of the placentals the 

 entire surface of the chorion is covered with a number 

 of short villi; these " chorion- villi " take the form of 

 pit-like depressions of the mucous membrane of the 

 mother, and are easily detached at birth. That 

 happens in most of the ungulata (the sow, camel, 

 mare, etc.), the cetacea, and the prosimise ; these 

 " malloplacentalia " (animals with a diffuse placenta) 

 have been denominated the indeciduata. The same 

 formation is present in man and the other placentals 

 in the beginning. It is soon modified, however, as 



