338 



COMPARATIVE ANATOMY 



moreover retained in the Monotremes, and even in Marsupials 

 the ova are relatively large as compared with those of the higher 

 Mammalia. 



As the amount of yolk gradually became reduced in the course 

 of phylogenetic development, close relations were set up between 

 the foetal (allantoic) and maternal blood-vessels, the allantois 

 becoming closely applied to the serosa to form a chorion (Fig. 

 271); but that this condition was only very slowly evolved is 

 shown by the fact that, even at the present day, Mammals exist m 

 which it has not been reached. These (viz., Monotremes and most 

 Marsupials) are therefore known as Aplaccntalia or Achoria, m 

 contradistinction to the higher Placcntcdin or Choriata. Moreover, 



in the Eodentia.Insectivora, Cheir- 

 optera, Carnivora, and Ungulata 

 more or less distinct indications 

 of an unibilical placenta, formed 

 in connection with the yolk-sac,, 

 can still be observed, and at a still 

 earlier stage the ova are nourished 

 by uterine lymph (compare p. 336). 

 In Monotremes and Marsu- 

 pials, both the yolk-sac and allan- 

 tois take part in respiration; in 

 the former the two are of equal 

 importance, while amongst the 

 latter the yolk-sac is solely or 

 mainly (Phalcolarctos) important 

 in this respect. In Perameles 

 obesula a further approach towards 

 the formation of a true allantoic 

 "placenta is seen, the allantois 

 giving rise to small vascular villi. 

 In most Marsupials the allantois 

 serves merely as a urinary reser- 

 voir, and in none of them does 

 it possess any important function 

 as an organ of nutrition, the young being born at a relatively early 

 stage, when they become attached to the teats of the mother, and 

 are then nourished by means of milk (see p. 288). 



In the higher Mammals, the umbilical placenta has usually 

 only a very temporary importance, though in some cases {e.g.^ 

 Rodents) it probably takes some part in respiration and nutrition 

 during the whole uterine life. The allantois extends out from the 

 body of the embryo and becomes attached to the serous membrane 

 to form the chorion, from which numerous villi extend into the 

 uterine wall (Fig. 271). As both the latter and the allantois become 

 extremely vascular, the uterine and allantoic capillaries and 

 sinuses coming into close contact with one another, a complicated 



Fig. 271. — Di.\gr-im of the Foetal 

 Membranes of a Placental 

 Mammal. (From Boas's Zoology.) 



at, allantois ; am, amnion ; 6, yolk- 

 sac (umbilical vesicle) ; the outer- 

 most line represents the serous 

 membrane. The outer wall of 

 the allantois has united with the 

 serous membrane to form the 

 chorion from which branchial villi 

 arise. 



