80 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 fcetal villi are received into corre- 
sponding depressions of the maternal surface, from which at the 
period of parturition they are simply withdrawn.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 placente are always non-deciduate. Recent observations by 
Sir W. Turner on the placentation of the Dugong show that the 
