UROGENITAL SYSTEM. 351 
mals the umbilical or navel cord, in which the blood-vessels run. In 
the mammals there are several variations from the above account of the 
development of the allantois, but they can be reconciled with the 
typical condition in the sauropsida. There are also several othet 
variations and the relations of allantois to the other structures is more 
complicated, but details and the many modifications must be ignored 
here, only an outline of the broader features being given. 
In the mammals there is the same fusion of allantois and serosa as 
in the sauropsida, the fused area here being called the chorion. On 
arrival in the uterus by way of the Fallopian tube, the egg becomes 
implanted in the uterine wall, and a little later, with the development 
of the chorion, villi are formed on the outer surface of the egg. These 
are invaded by the chorionic blood-vessels and they branch and extend 
into depressions or crypts in the walls of the uterus. The latter become 
very vascular, the blood spaces of the maternal tissue enveloping the 
villi with only the thinnest of walls between the vessels of the mother 
and those of the young. (There is never any actual connexion between 
the blood-vessel of parent and embryo and so blood corpuscles cannot 
pass from one to the other. All that takes place is largely of the nature 
of osmosis—solutions of gases, of nourishing substances and of nitrog- 
enous waste passing from one to the other. There is difficulty in 
explaining the passage of proteids and fats.) This structure, consisting 
of the allantoic derivatives of the embryo and the mucous lining of the 
uterus, is known as the placenta. 
In the monotremes and in most marsupials no placenta is formed, 
but it has been recently shown that a true placenta occurs in a few of the 
latter group. In other mammals a placenta always occurs, the struc- 
tures presenting many forms, but these.may be grouped under a few 
heads. (It must be borne in mind that this classification is purely 
morphological and does not necessarily imply close relations of the 
species included or identity of method of formation.) 
In many mammals, at the time of birth, the maternal and embryonic 
parts of the placenta simply separate, only the latter passing away 
with the young. These are called non-deciduate placente. In 
the others the union of the foetal and the maternal tissues is so intimate 
that the inner surface of the uterus is included in the afterbirth. These 
form the deciduate type. The non-deciduata include two divisions. 
In the diffuse placente (edentates, whales, perissodactyls, many 
artiodactyls) the villi are distributed over the entire surface of the 
