THE RISE OF THE MAMMALIA 129 



the yolk contained in the yolk-sac, and that of the 

 Eutherian Mammal, which is born in an advanced 

 state of development after being nourished in the 

 mother by means of the allantoic placenta. 



The Marsupial embryo is nourished in the uterus 

 of the mother almost entirely by the yolk-sac, which 

 contains food material itself and also comes into 

 contact with the uterine wall of the mother and 

 forms a poorly developed yolk-sac placenta. This 

 mode of nutrition or gestation lasts for only eight 

 days or so, when the foetus is delivered. 



Now the remarkable fact is that in the Marsupial 

 foetus an allantois is actually formed, growing out 

 from the hind gut in a typical manner; but soon after 

 its formation, instead of growing out and fusing 

 with the trophoblast and the uterine wall, it begins 

 to shrivel away and its blood-vessels atrophy and 

 disappear. In only one Marsupial, the Bandicoot 

 (Perameles), does the allantois succeed in reaching 

 the wall of the maternal uterus and forming a small 

 true placenta, which acts as a feeble source of nutri- 

 ment for a few days (Fig. 25). It is evident that we 

 are dealing here with a degenerate condition. The 

 allantois begins to develop, but -in the majority of 

 cases it degenerates before it has performed any 

 function at all. This certainly would not be the case 

 if it were in a primitive condition, representing the 

 origin of the placenta, because in that case it would 



S. A. K. 9 



