REPRODUCTIVE ORGANS 79 
birds, rather than from the uterine walls by means of the allantoic 
vessels, as in the higher mammals. The latter vessels, in fact, play 
even a much less important part in the development of these 
animals, not only than in the placental mammals, but even than in 
the Sauropsida, for they can scarcely have the respiratory function 
assigned to them in that group: pulmonary respiration and the 
lacteal secretion of the mother very early superseding all other 
methods of providing the due supply both of oxygen and of food 
required for the development and growth of the young animal. 
In this sense the Marsupials may be looked upon as the most 
typically “‘mammalian” of the whole class. In no other group do 
the milk-secreting glands play such an important part in pro- 
viding for the continuity of the race. 
In the third primary division of the Mammalia, the so-called 
Placentalia, the umbilical vesicle generally does not quite unite 
with the chorion, and disappears as development proceeds, so that 
no trace of it can be seen in the membranes of an advanced 
embryo; but it may persist until the end of the intra-uterine life 
as a distinct sac in the umbilical cord, or lying between the 
allantois and amnion. The disappearance or persistence of the 
umbilical vesicle does not, according to our present knowledge, 
appear to be correlated with a higher or lower general grade of de- 
velopment, as might be presupposed. It is stated to have been 
found in Man even up to the end of intra-uterine life, and also in 
the Carnivora, while in the Ungulata and Cetacea it disappears at 
an earlier age. In many, if not all, of the Rodentia, Insectivora, 
and Chiroptera, it plays a more important part, becoming adherent 
to a considerable part of the inner surface of the chorion, to which 
it conveys blood-vessels, although villi do not appear to be developed 
from the surface of this part, as they are on the portion of the 
chorion supplied by the allantoic vessels. These orders thus 
present to a certain extent a transitional condition from the Mar- 
supials, although essentially different, in possessing the structures 
next to be described. 
The special characteristic of the whole of the placental mammals 
constituting the majority of the class, is that the allantois and its 
vessels become intimately blended with a smaller or greater part of 
the parietes of the ovum, forming a structure on the outer surface of 
which villi are developed, and which, penetrating into corresponding 
cavities of the “decidua,” or soft, vascular, hypertrophied. lining 
membrane of the uterus, constitutes the placenta. This organ may 
be regarded, as Sir William Turner says, both in its function and in 
the relative arrangement of its constituent textures, as a specially 
modified secreting gland, the ducts of which are represented by the 
extremities of the blood-vessels of the fetal system. The passage 
of material from the maternal to the foetal system of vessels is not 
