So GENERAL ANATOMICAL CHARACTERS 



a simple percolation or diffusion through their walls, but is oc- 

 casioned by the action of a layer of cells derived from the maternal 

 or uterine structures, and interposed between the blood-vessels of 

 the maternal part of the placenta and those of the villi covering 

 the chorion, in which the embryonic vessels ramify. 



The numerous modifications in the details of the structure of 

 this organ relate to augmenting the absorbing capacity of the vessels 

 of the chorion, and are brought about either by increasing the com- 

 plexity of the foetal villi and maternal crypts over a limited area, 

 or by increasing the area of the part of the chorion covered by the 

 placental villi, or by various combinations of the two methods. 



The first class of variations has given rise to a distinction into 

 two principal kinds of placenta: (1) simple or non-deciduate, and 

 (2) deciduate. In the former the foetal villi are received into corre- 

 sponding depressions of the maternal surface, from which at the 

 period of parturition thej r are simply withdrawn. In the second, 

 or more complex form, the relation is more intimate, a layer of 

 greater or less thickness of the lining membrane of the uterus, 

 called " decidua," becoming so intimately blended with the chorion 

 as to form part of the placenta proper, or that structure which is 

 cast off as a solid body at parturition. In other words, in the one 

 case the line of separation between the placenta and uterus at birth 

 takes place at the junction of the foetal and maternal structures, in 

 the other through the latter, so that a portion of them, often of con- 

 siderable thickness, and containing highly organised structures, is 

 cast off with the former. It was once thought that the distinction 

 between these two forms of placentation is so important as to con- 

 stitute a sufficiently valid basis for a primary division of the pla- 

 cental mammals into two groups. It has, however, been shown 

 that the distinction is one rather of degree than of kind, as inter- 

 mediate conditions may exist, and it is probable that in different 

 primary groups the simpler, non-deciduate form may have become 

 developed independently into one or other of the more complex 

 kinds. 



Apart from its intimate structure, the placenta may be met with 

 of very varied general form. It may consist of villi scattered more 

 or less regularly over the greater part of the surface of the chorion, 

 the two extremities or poles being usually more or less bare. This 

 form is called the " diffused placenta." It is probably a primitive 

 condition, from which most of the others are derived, although its 

 existence must presuppose the absence of the umbilical vesicle as a 

 constituent of the chorionic wall. It is found at present in the 

 Manis among Edentates, the Cetacea, the Perissodactyle Ungulates, 

 and the Camels, Pigs, and Chevrotains among the Artiodactyles. 

 Such placentae are always non-deciduate. Kecent observations by 

 Sir W. Turner on the placentation of the Dugong show that the 



