AND AFFINITIES OF TARSIUS. 



485 



this view of its significance is correct is clearly shown by its mode 

 of development in Tattosia, and by its occurrence in the Marmoset 

 (Hapcde) in a condition which can best be described as semi- 

 vesicular. In Tarsms, its primordium is constituted at first, 

 according to Hubrecht, simply b}^ the unsplit proximal portion 

 of. the axial mesoderm proliferated from the hinder margin of 

 the embrj'^onal ectoderm ; if we are justified in regarding this 

 mesoderm as precociously formed primitive streak mesoderm, 

 then the connecting stalk of Tarsius can also be brought into liiae 

 with what we know of the development of the allantois in lower 

 Ma-mmals. The predisposing fa,ctor in the evolution of the 

 connecting stalk in Tatusia and Tarsms is doubtless to be sought 

 in the placental attachment which is early established hj the 

 trophoblast situated just behind the embryonal area and in 

 immediate proximitj^ to the normal seat of origin of the allantois. 

 As to the development of the connecting stalk in the Anthro- 

 poids, we have no certain knowledge. It seems jsrobable, however, 



Text-figure 5. 



est 



ainn. 



eoe. 



S.c. 



Diao-ram to show tlie structure of tbe earlj- blastoc,yst in the Anthropoidea. Note 

 the embryonal ectoderm (cross-lined) forming the floor of the amnio-embryonal 

 vesicle {amn. amnion), and the underlying yolk-sac vesicle (i/s-c.) ; the con- 

 necting stalk (est.) with the allantoic duct (all.cl.) ; and the extensive extra- 

 embryonal coelom (e.r.coe.) bounded by the chorion {cli.). 



that it does not arise in the same way as in Tarsms, but that, to 

 begin with, it is simply formed by the persistent hinder portion 

 of^the layer of extra-embryonal mesoderm which in the early 

 blastocyst lies between the amnio-embryonal vesicle and the 

 trophoblast. Through the extension of the ctelom, this layer 

 would seem to become split into chorionic mesoderm on the 

 outside and amniotic mesoderm on the inside, except posteriorly, 



33* 



