124 EDWARD MCCRADY, JR. 



Comparative notes. With the exception of the precocious 

 organs already listed (forelimb, lungs, etc.) the stage 29 

 opossum resembles very closely a 10-day rabbit (Minot's 

 no. 562), a 3-day chick (Patten, fig. 43), a 1.4-mm. lizard 

 (Peter's no. 74). The pancreas and allantois were not dis- 

 cussed in chapter IX, and need special comment. 



In comparison with the corresponding organs in placental 

 mammals, the pancreas in the opossum is precocious and the 

 allantois is retarded. In both of these respects the opossum 

 is more like the lizard and the chick. But in the case of the 

 allantois the opossum is the least advanced of all the animals 

 studied. 



The placental mammal develops his allantois early and 

 constructs from it a mechanism by which he can take ad- 

 vantage of his mother's pancreas, and lungs. The lizard and 

 the chick use the allantois for respiratory purposes, but need 

 the pancreas to enable them to utilize some of the more com- 

 plicated constituents of the yolk. It is not likely that the 

 opossum uses his pancreas in the utilization of uterine milk, 

 .but he needs it at an early stage for another reason. Even if 

 the allantois should grow out and fuse with the serosa, it 

 would not find any richer source of oxygen than that already 

 found by the well-developed yolk sac circulation. The yolk 

 sac thus serves as the source of both food and oxygen; and 

 the allantois, which becomes completely surrounded by it, 

 serves only as a urinary reservoir. 



Apparently the need of the opossum foetus which is least 

 well served by this arrangement is that of oxygen. And it 

 may be that the three most precocious features of opossum 

 development the development of the lungs, the development 

 of the forelimbs, and the act of parturition are all primarily 

 adapted to fill that need. The act of parturition carries the 

 foetus to a rich source of oxygen, which the precocious lungs 

 can take advantage of. But this act also necessitates the 

 finding of a new source of food. The precocious forelimbs, as 

 will be seen later, carry him to the new source of food. The 

 well-developed pancreas then makes the utilization of this 

 new food possible. 



