REPRODUCTIVE ORGANS 77 
some points which require notice, as peculiar to the mammalian 
class, and as affording at least some hints upon the difficult subject 
of the affinities and classification of the members of the group. 
The nourishment of the fetus during intra-uterine life takes 
place through the medium of certain structures, partly belonging 
to the foetus itself and partly belonging to the inner parietes of the 
uterus of the parent. These in their complete form constitute the 
complex organ called the “placenta,” serving as the medium of 
communication between the mother and foetus, and in which the 
physiological processes that are concerned in the nutrition of the 
latter take place; but, as we shall see, though a placenta, in the 
usual acceptation of the term, is peculiar to the mammalian class, it is 
not in all of its members that one is developed. The structures to 
which we shall have especially to refer are the outer tunic of the 
ovum, to which, however formed, the term “chorion” is commonly 
applied, and two sac-like organs connected with the body-cavity of 
the embryo, both formed from the splanchnic mesoblast, lined by a 
layer of the hypoblast. These are the “umbilical vesicle” or “ yolk- 
sac” and the “allantois.” 
The umbilical vesicle is a thin membrane enclosing the yolk, 
which by the doubling in of the ventral walls of the embryo becomes 
gradually formed into a distinct sac external to the body, with a 
pedicle (the omphalo-enteric duct) by which for a time a communica- 
tion is maintained between its cavity and the intestinal canal. In 
the walls of this sac blood-vessels (omphalo-meseraic or vitelline) 
are developed in connection with the vascular system of the embryo, 
through which, either by their contact with the outer surface of the 
walls of the ovum, or by the absorption through them of the 
contents of the yolk-sac, the nutrition of the embryo in the lower 
vertebrates chiefly takes place. In mammals the umbilical ves- 
icle plays a comparatively subordinate part in the nourishment 
of the foetus, its function being generally superseded by the 
allantois. 
The last-named sac commences at a very early period as a 
diverticulum from the hinder end of the alimentary tract of the 
embryo. Its proximal portion afterwards becomes the urinary 
bladder, the contracted part between this and the cavity of the 
allantois proper constituting the urachus, which passes out of the 
body of the feetus at the umbilicus together with the vitelline duct. 
The mesoblastic tissue of the walls of the allantois soon becomes 
vascular ; its arteries are supplied with foetal blood by the two 
hypogastric branches of the iliacs, or main divisions of the abdominal 
aorta, and the blood is returned by venous trunks uniting to 
form the single umbilical vein which runs to the under surface of 
the liver, where, part of it joining the portal vein and part entering 
the vena cava directly, it is brought to the heart. These are 
