Xect. III.] EMBRYO OF OPOSSUM. 61 



Marsupials that source of supply exists for a very short 

 time, and then the mammary glands furnish the table. 

 So also do they in the high types of mammals, but 

 not until after a very long interval. In these the food- 

 yolk is extremely smaU in quantity, yet the embryo 

 is not supplied with milk when that ceases, but receives 

 its nourishment through the instrumentality of an 

 -enormous allantois, united with the highly vascular 

 walls of the dilated oviduct. 



But even in the Eutheria there is a great variation 



Fig. 7. — Uterine embryo of Virginian Opossum (Didelphys virrjiniana), 

 magnified 6 diameters. 



lis to the time at which this last and newest mode of 

 feeding an embryo comes into play, some kinds having 

 tender, and others precocious, young. 



The observations of Professor Chapman of America 

 show that these bags are aU present in the Kangaroo, 

 but that they are all small and arrested, so that neither 

 the allantois, as in the higher mammals, nor the yolk- 



