1164 PUOF. J. P, HILL AND MR. R. H. BURNE ON THE 



We must confess we are quite unable to accept the views of 

 these two authorities, either in respect of the secondary nature of 

 the diffuse placenta of the Lemurs or in regard to its dis- 

 similarity to the diffuse placenta of other Mammals. It appears 

 to us they have attempted to read the story of placental 

 evolution the Avrong way round. 



In our opinion, any attempt to trace the evolution of mam- 

 malian placental arrangements must be based on a consideration 

 of the probable conditions which obtained at the time viviparity 

 replaced oviparity. Knowing the arrangement of the foetal 

 membi'anes in the existing Monotremes, we can picture the 

 conditions with reasonable certainty. We snould postulate, 

 then, that the common ancestral stock, from which the Di- 

 delphia and Motiodelphia diverged, possessed an arrangement 

 of the fo3tal membranes similar to that of the existing Mono- 

 tremes and of PhasGolarctos amongst the Marsupials — i. e., the 

 outer wall of the embryonal formation consisted as to one-half 

 or thereabouts of unsplit blastocyst wall or omphalopleure, in 

 part vascularised by the vitelline A^essels of the vascular area 

 of the yolk-sac, and as to the remainder, of allanto-chorion, 

 vascularised by the umbilical vessels of the vesicular allantois. 

 The uterine mucosa possessed a persistent covering epitheliurxi, 

 functional uterine glands, and a rich vascular supply. 



These being the structural elements involved, the primitive 

 placenta was constituted simply by the close apposition of the 

 omphalopleure and the allanto-chorion with the vascular lining 

 of the uterus. Here we have the simplest possible type of 

 placenta, partly allantoic, partly vitelline and diffuse in the 

 sense that the whole of the outer surface of the embryonal 

 formation is involved in the carrying on of the exchanges 

 between the maternal and foetal blood-streams. The trof)hoblast 

 covering the entire outer surface is as yet a simple layer, absorp- 

 tive but not actively phagocytic. 



Out of the primitive placenta here outlined, we see no difficulty 

 in deriving on the one hand, the relatively simple placental 

 arrangements characteristic of the Marsupialia and on the 

 other, the varied types of placenta characteristic of the Mono- 

 delphia, the non-deciduate diffuse type representing only a 

 relatively slight elaboration of the priixiitive form, and the 

 varieties of the " deciduate " type i-esulting from the assumption 

 by the trophobla,st of active phagocytic properties, the precise 

 characters assumed by the placenta being in all cases determined 

 by a variety of conditions, intrinsic and environmental or uterine. 

 We therefore remain firm in our belief that the placentation of 

 the Lemurs is genuinely simple and little specialised. 



Finally, as concerns the reputed distinction between the 

 Lemiu-oid diffuse placenta and that of other Mammals, we 

 readily grant there are differences in detail, but we quite fail 

 to see that there is any fundamental difference, or even enough 

 difference to justify the calling of the former " pseiido-plicate '"' 



