THE PLACENTATION OF PEEAMELES. 437 



opiiiiou only be regarded as conclusive in favour of the view 

 that from such a condition of fcBtal membranes as is common 

 to these two types, Perameles and Erinaceus, phylogenetic 

 speculation on the placenta must start. We would, therefore, 

 attach very special phylogenetic importance to the non-separa- 

 tion in marsupials of the vascular omphalopleure into yolk-sac 

 splanchnopleure and somatopleural chorion. This non-sepa- 

 ration, ensuring, as Semon (8) has pointed out, the retention 

 of the vessels of the vascular area in a superficial position, 

 eminently suited for the performance of nutritive and respira- 

 tory functions, we can only regard as a physiological adapta- 

 tion, and as probably the first to have been adopted on the 

 initiation of uterine development in the common ancestors of 

 the Metatheria and Eutheria. This condition is not " pro- 

 bably a purely marsupial modification/^ as Minot would have 

 us believe (6, p. 129), for it is undoubtedly also manifested in 

 the occurrence in certain lowly Eutherians of a temporary 

 yolk-sac placenta preceding the formation of the definitive 

 allantoic one. As Hubrecht has shown in Eriuaceus (9), it is 

 only after the allantoic placenta has taken the place of the 

 omphalopleural that the mesoderm of the vascular omphalo- 

 pleure splits into splanchnic and somatic layers, and this 

 delaying of the splitting process Hubrecht (20) attributes to 

 the vital importance of the yolk-sac placenta. 



It is not necessary to dilate on the significance of the dis- 

 coid form of the allantoic placenta of Perameles. " On the 

 Balfourian hypothesis,'^ as Professor G. B. Howes has pointed 

 out, the view that the discoidal type of allantoic placenta is 

 the most primitive " is by far the most natural one warranted 

 by the facts.'^ ^ 



Univebsitt of Sydney, N.S.W. ; 

 April IGtl), 1897. 



1 ' Nature,' vol. xl, p. 420. 



