520 MAMMALIA. 



constitute the embryonic membrane called the chorion (Fig. 271 

 ft, ch). The chorion develoj)s vascular villi which enter into close 

 relation with the uterine wall. In this manner there is de- 

 veloped a relatively large surface, permeated with branches from 

 the foetal vessels, the blood of which is in intimate osmotic 

 connection with the blood of the uterine wall. This connection 

 of the chorion of the foetus with the uterijie walls gives rise to 

 the placenta, by means of which the nourishment and respiration 

 of the foetus are provided for in the body of the mother. The 

 placenta is wanting only in the Monotremata and most Mar- 

 supialia, which, therefore, have been called Aplacentalia, as 

 opposed to the rest of the Mammalia, which have a placenta and 

 have been called Placentalia. The placenta presents great varia- 

 tions in the individual orders, in its special development and in 

 the mode of its connection with the uterine walls. Either the 

 villi of the placenta are loosely connected with the uterine walls, 

 and separate from the latter at birth (Adeciduata), or they be- 

 come so intimately united with the uterine mucous membrane 

 that the latter comes * away with the embryo at birth, as the 

 decidua or after-birth (Deciduata). In the first case the villi 

 may be numerous and uniformly distributed over the whole 

 chorion (diffuse placenta of Ungulata, Cetacea, etc.), or be 

 aggregated in special places, forming small tufts, the so-called 

 cotyledons (ruminants). In the other case, the placenta with 

 its villi is confined either to an annular zone on the chorion (pi. 

 annularis or zonary placenta of Carnivora, Pinnipedia), or to 

 a discoidal area (metadiscoidal placenta of man and apes, dis- 

 coidal placenta of rodents, insectivores, bats). 



The trophoblast of the mammalian embryo is a structure of 

 great physiological importance. It is the layer in relation with 

 the uterine wall, and in the early stages is employed in secreting 

 fluid into the cavity of the blastodermic vesicle and so facili- 

 tating the nourishment of the embryo in the earliest stages. 

 Later, it is the layer by which the attachment of the embryo to 

 the uterine wall is effected. It becomes much thickened and 

 vascularised from the allantoic vessels, and its outer wall be- 

 comes phagocytic and in many cases eats its way into the uterine 

 wall, so that the embryo actually becomes imbedded in the 



* In some cases a portion of the foetal tissues remains in the uterus and 

 is absorbed (mole, etc.). 



