PLACENTA IN PERAMELES 169 



a structure by wliicli the jihysiological processes intended for 

 the well-being of the embryo can be carried out. 



Such a conception makes no allowance whatever for the 

 work of the bilaminar omphalopleure of marsupials — itself 

 non-vascular but physiologically of considerable importance. 



Even less acceptable is the suggestion of Professor Hubrecht 

 (1909), who insists that ' fusion of embryonic with maternal 

 tissue is a conditio sine qua non, and so we must admit 

 a placenta in the case of Didelphia (Perameles) and deny it to 

 certain Monodelphia (Equus, Sus, Nyticebus, Galago, and 

 others) '. As Assheton has pointed out, under this scheme 

 * the sheep is a placental, a cow a non-placental mammal \ 



Eeaction between mother and embryo, or rather dependence 

 of the latter on the mother for food, oxygen, and the removal of 

 its waste products, may be said to commence from the time of 

 the first appearance of the ovum in the uterus. ' The mam- 

 malian ovum,' says Hill (1910, p. 113), ' already in the mono- 

 tremes greatly reduced in size as compared with that of reptiles, 

 and quite minute in the Metatheria and Eutheria, con- 

 tains within itself neither the cubic capacity nor the food 

 material necessary for the production of an embryo on the 

 ancestral reptilian lines. We accordingly find that the primary 

 object of the first developmental processes in the mammals 

 has come to be the formation of a vesicle with a complete 

 cellular wall capable of absorbing nutrient fluid from the 

 maternal uterus.' 



Our knowledge of the physiology of the early stages of 

 mammalian intra-uterine development is admittedly as yet 

 very incomplete. Nevertheless, it is quite certain that the 

 amount of nutrient material present in the ovum is absolutely 

 insufficient for even the most elementary developmental pro- 

 cesses, and has to be supplemented, from the very beginning, 

 from outside sources. 



Even a cursory consideration of the above will serve to 

 indicate that in the uterine development of the viviparous 

 mammals there occur two distinct phases, differing entirely 

 in the means by which the necessary physiological processes 



