296 MAMMALIA. 



pellucida), round which a layer of albumen is often deposited in the 

 oviduct. 



The fertilization and total segmentation * of the ovum always take 

 place in the oviduct (Fallopian tube). Amnion and allantois are 

 present in Mammalia. In the uterus the ovum acquires a villous coat 

 (chorion), derived from the original zona and from the subzonal mem- 

 brane (so-called serous envelope), which is developed within the zona. 

 It becomes attached to the uterine wall by means of the chorion (fig. 

 679). Later on, the peripheral part of the allantois also becomes 

 applied to the chorion, and, as a rule, penetrates with its vessels into 

 the villi (secondary chorion), so that there is developed a relatively 

 large surface, permeated with branches from the foetal vessels, 

 the blood of which is in intimate endosmotic connection with the 

 blood of the uterine wall. This connection of the allantois and 

 chorion of the foetus with the uterine walls gives rise to the Placenta, 

 by means of which the nourishment and respiration of the foetus are 

 provided for in the body of the mother. The placenta is wanting 

 only in the Monotremata and Marsupialia, which, therefore, are 

 known as Aplacentalia, as opposed to the rest of the Mammalia, 

 which have a placenta, and are called Placentalia. The placenta 

 presents great variations in the individual orders, in its special 

 development and in the mode of its connection with the uterine walls. 

 Either the villi of the placenta are loosely connected with the 

 uterine walls, and separate from the latter at birth (Adeciduata), or 

 they become so intimately united with the glands of the uterine 

 mucous membrane that the latter comes away with the embryo at 

 birth, as the decidua or after-birth (Deciduata).^ In the first case 

 the allantois may grow completely round the ovum, and the villi be 

 numerous and uniformly distributed over the whole chorion (diffuse 

 placenta of Ungulata, Cetacea), or be aggregated in special places, 

 forming small tufts, the so-called cotyledons (Ruminants). In the 

 other case, the placenta with its villi is confined either to an annular 

 zone on the chorion (PL annularis, or zonary placenta of Carnivora, 

 Pinnipedia), or to a discoidal area (discoidal placenta of Man, 

 Apes, Rodents, Insectivores, Bats). 



* [According to Caldwell's recent discovery, which was communicated to the 

 British Association at Montreal in September of the present year (1884), but 

 of which no details have as yet come to hand, the Monotremata are oviparous 

 and their ova meroblastic.] 



f [For a fuller account of the structure and development of the various kinds 

 of placenta, the reader is referred to Balfour's Comparative Embryology, vol. ii., 

 p. 193.] 



