478 ON THE ZOOLOGICAL POSITION 



Primates may quite easily have originated as the result of 

 adaptive specialis.ition iu the coui-se of evolution. Hubrecht, 

 however, took an entirely opposite view and held that the 

 placentation of these forms is not genuinely primitive, but had 

 arisen from some hypothetical, more complicated form of 

 placenta as the result of secondary reduction and degeneration ; 

 and Assheton, inclining to the same point of view, suggested 

 that it might have been derived from a primitive Carnivore 

 type, also l)y reduction. For these views, expressed in con- 

 nection with particular theories of placental evolution, there is, 

 so fa.r a,s i am aware, not the slightest direct evidence. 



On the other hand, the evidence derivable from a study of 

 the development of the fatal membranes seems to me to provide 

 us with a perfectly definite lead. Hubrecht's observations on 

 ISfycticehus show that the allantois grows out into the extra- 

 embryonal Cffilom as a small stalked vesicle, just as it does in all 

 primitive Mammals. It rapidly increases in size and already in 

 the embryo of 4*2 nun. (G. L.) has extended over and fused with 

 the discoidal area of chorion, which is thus transformed into 

 vascular allanto-chorion (text-fig. 1, cdl.ch.). In later stages, 

 following the splitting of the extra-embryonal mesoderm, it 

 rapidly spreads and the chorion, over its entire extent, is 

 converted into vascular allanto-chorion. With this rapid and 

 very marked growth of the allantois are to l>e correlated 

 two other occurrences to wliich I would direct attention : 



(1) the extra-embrj'onal coelom rapidly extends throughout the 

 entire extent of the mesoderm of the blastocyst wall or omjDhalo- 

 pleure, which thus becomes split into chorion and yolk-sac wall ; 



(2) the yolk-sac becomes established .as an independent vesicle, 

 a,nd the yolk-sac placenta, if such temporarily exists in con- 

 nection with the early embryo, becomes completely replaced by 

 the allantoic. 



These development features in the Lemuroids — viz., the 

 establishment of a complete chorion and its early vascularization 

 by the rapidly growing allantois, the early formation of an 

 extensive extra-embiyonal coelom and the separation of the 

 yolk-sac as an independent vesicle — seem to me to foreshadow 

 in the most uiniiista.ka.ble way cori'esponding and highly 

 cha,ra.cteristic events in the early development of both Tarshis 

 and the Anthropoidea. We have only to suppose these onto- 

 genetic happenings in the Lemuroids telescoped into still earlier 

 stages as the result of developmental adaptation in order to 

 reach the structural conditions charactei-istic of the early 

 blastocysts of the other Primates, and for such adaptations 

 to become perfected, ample time has been aA'aiJable, seeing 

 that the Lemiu^oids and the Tarsioids were already well 

 differentiated from each other in the Lower Eocene. 



From the standpoint of embryology, I would accordingly 

 range myself with those Avho, on comparative anatomical and 

 palfeontological grounds, see iji the existing Lemuroids the 



