384 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



account will give the major heads under which the different types may 

 be arranged. 



In many mammals there is no very intimate connexion between 

 the allantoic and the maternal tissues, and when birth occurs there is 

 merely a separation between maternal and foetal parts. This divi- 

 sion was formerly called non-decidnata, for which, for reasons not 

 necessary to discuss here, the term semiplacenta has recently been 

 introduced In two marsupials {Dasyurus and Peromeles) this type 

 occurs and in them the outer surface of the chorion remains smooth, 

 the only union between maternal and foetal structures being merely 

 the apposition of the two surfaces. A placenta of this sort is the 

 semiplacenta avillosa. 



In all other mammals the surface of the chorion becomes covered 

 with projections (villi) into which loops of the allantoic blood-vessels 

 extend, thus increasing the amount of absorptive surface. These 

 villi are arranged in different ways. Thus in the whales, lemurs, and 

 many ungulates they are uniformly distributed over the surface, and 

 extend into the glands of the uterus, a layer of modified uterine 

 epithelium intervening between villi and maternal blood-vessels 

 (semiplacenta diffusa). In other ungulates (most ruminants) the 

 villi are arranged in groups or cotyledons, the chorionic surfaces 

 between the cotyledons being smooth (semiplacenta diffusa, the old 

 cotyledonary placenta). In the dugong the villi form a ring or zone 

 around the chorion, either end being free from them (semiplacenta 

 zonaria). 



In the remaining mammals the union of uterine and foetal struc- 

 tures is far more intimate. The uterine epithelium disappears early, 

 and the villi extend into the remains of the glands, so that foetal and 

 maternal tissues are inextricably intertwined. In these cases, when 

 birth occurs, not only is the allantoic portion of the placenta cast off, 

 but the mucosa splits and is lost with the other. The forms having 

 this type were formerly classed as deciduata, now they are said to have 

 a true placenta (placenta vera) composed of foetal and maternal 

 portions. Of these there are two types with an intermediate con- 

 dition, these being based as before on the distribution of the villi. 

 In the zonary placenta (most carnivores) the villi form a ring around 

 the chorion, and in the elephant and the Hyracoids cotyledons also 

 occur. The second type, represented by bats, insectivores, rodents 

 (fig. 406) and the higher primates (including man) the placenta is 

 disc-like, occurring on only one side of the chorion (discoidal pla- 



