438 COMPARATIVE ANATOMY 



placenta become extremely vascular, and thus the necessary inter- 

 change of materials can take place between the blood of the em- 

 bryo and mother. In Trachydosaurus and Cyclodus, as well as in 

 the Chelonia, a kind of umbilical placenta is apparently also 

 formed. 



The fact that a vascular yolk-sac (often known as the umbilical 

 vesicle) is present in all Mammals, indicates that they are de- 

 scended from forms in which, as in the Sauropsida, the eggs were 

 rich in yolk, and which were viviparous. This condition is more- 

 over 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 was gradually 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. 

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

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

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

 Marsupials) are therefore known as Aplacentalia or Achoria, in 

 contradistinction to the higher Placentalia or Choriata. Moreover, 

 in the Rodentia, Insectivora, Cheiroptera, Carnivora, and Ungulata 

 more or less distinct indications of an umbilical placenta, formed 

 in connection with the yolk-sac, can still be observed, and at a 

 still earlier stage the ova are nourished by the uterine lymph. 



In Monotremes and Marsupials, both the yolk-sac and allantois 

 take part in respiration ; in the former the two are of equal 

 importance, while amongst the latter the yolk-sac is solely (e.g. 

 Dasyurus) or mainly (e.g. 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 reservoir, 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 

 (cf. p. 375) : the form of the mouth and also that of the teats is 

 especially adapted to this end. 



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. 333). 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 

 allantoic placenta arises, consisting of maternal and foetal parts 



