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 foetus during intra-uterine life takes 

 place through the medium of certain structures, partly belonging 

 to the fcetus 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 fcetus, 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 of the fcetus, its function being generally superseded by the 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 fcetus 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 



