REPRODUCTIVE ORGANS 79 



birds, rather than from the uterine Avails by means of the allantoic 

 vessels, as in the higher mammals. The latter vessels, in fact, play 

 oven 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 foetal system. The passage 

 of material from the maternal to the foetal system of vessels is not 



