PLACENTA IN PEKAMELES 127 



splanchnic mesoderm of the allantois fuses, and the vessels 

 ramifying on the surface of the allantois thus come to lie 

 immediately below the layer of true chorion. Up to this point 

 the development of the placenta may be regarded as being fairly 

 typical, but, from now on, the development of this organ, 

 according to Hill's account, results in the appearance of such 

 structural peculiarities and modifications as to give rise to the 

 general impression that the placentation of Perameles, 

 at any rate as concerns its more intimate development, is 

 without parallel in the whole mammalian group. 



The allantoic placenta is completed ' by the gradual degenera- 

 tion and resorption of the enlarged chorionic ectoderm cells 

 over the placental area proper. These cells thus take no further 

 share in placental formation.' The result of this is the close 

 apposition of maternal and foetal blood-vessels, the two blood- 

 streams being ' now onl}' separated by their thin endothelial 

 walls and perhaps a thin layer of syncytial protoplasm ' (p. 388). 

 A 3^olk-placenta is present, formed by the close apposition 

 of the vascular area of the yolk-sac to the highly vascular 

 uterine syncytium outside the allantoic placental area. 



It is not necessary at this stage to enter into any discussion 

 as to the significance of the presence of an allantoic placenta 

 in Perameles other than to indicate that it has been 

 definitely accepted by most embryologists, that there is now 

 no reason to doubt the common origin of Metatheria and 

 Eutheria from a primitive placental stock. Hill says it is 

 ' exceedingly improbable that an allantoic placenta should 

 have been twice independently acquired and in such a funda- 

 mentally similar manner v.ithin the limits of the mammalian 

 class ' (p. 433). 



Now, outside the intrinsic interest of the presence of an 

 allantoic placenta in a member of a group formerly regarded 

 as aplacental, it would be expected, on a priori grounds, 

 that the occurrence would be of importance in elucidating the 

 phylogeny of the mammalian placenta or, at least, in giving 

 us some means of arriving at a definite idea of the method of 

 placental formation in the original protoplacental group. 



