STUDIES IN MAMMALIAN EMBETOLOGY. 529 



the Didelphia, none in the Monodelphia; and the very large 

 size (when compared to the embryonic area) to which the 

 blastocyst of the higher Mammalia increases has generally 

 been looked upon as a repetition, called forth by heredity, of 

 these ancestral lecithophorous arrangements. To me it has 

 always appeared that this explanation is rather strained. A 

 yolk-sac without yolk would be an encumbrance to a mammal 

 that completed its development inside the maternal genital 

 ducts, and would long since have been reduced or even eliminated 

 by natural selection — unless under the changed circumstances 

 a new and important function has come to be fulfilled by it, 

 which is of equally vital importance to the continuation of the 

 species. 



This has, I hold, been the case in Mammalia. When the 

 nutritive contents of the yolk-sac were no longer of primary 

 importance, and a considerable reduction in size of the blasto- 

 cyst might have gone hand in hand with the change from 

 mesoblastic to holoblastic segmentation, this was not effectuated 

 because another factor came into play. 



The vascular area which heredity called forth on the surface 

 of the yolk-sac, and by the aid of which the nutritive contents 

 of that sac were elaborated and absorbed, must have rendered 

 eminent service for the establishment of a different mode of 

 nutrition as soon as the embryo underwent a considerable 

 part of its development inside the maternal generative ducts. 

 The beautiful figures which Selenka has given for the opossum 

 (27) demonstrate this most forcibly, and the temporary abdi- 

 cation of the allantois in this particular case is also most 

 instructive. Now, for a satisfactory working of the new 

 arrangement it is undoubtedly of the utmost importance that 

 the surface of the area vasculosa should be stretched to its 

 maximum extent, and at the same time should be elastic 

 against pressure tending to throw it into folds. The change 

 required would thus be the substitution of liquid contents 

 serving the purpose just alluded to, instead of the nutritive 

 contents characteristic of the Hypotherian ancestors. With 

 the absorption and retention of this liquid, under a certain 



