AND AFFINITIES OF TARSIUS. 



481 



the middle line, from the hinder margin of the embryonal 

 ectoderm. Hubrecht compared it with the ventral mesoderm 

 of the Amphibia, but, without entering into that question, 

 what I Avant to suggest now is that it is none other than a 

 precociously formed part of the primitive streak mesoderm * — 



Text-fio'ure 3. 



hM. 



errii 



hr. ecto 



amn. 



amn 



-s.c. 



•~f~mes. 



ejc. coe.^% 



Tamil's spectrum. Diagram (after Hubrecht ) to show the arrangement of the fu3tal 

 membranes, prior to the closure of the amnion. Note in particuhir, the j'olk- 

 sac (^-s.c.) now established as an independent vesicle ; the connecting stalk 

 (csi.) into the proximal half of which extends the allantoic duct (all.d.) ; the 

 placenta (PI.) ; the thick laj'er of loose mesoderm (mes.) into which the extra- 

 embryonal coelom does not extend with the result that the distal portion of the 

 connecting stalk does not become separated from the chorion ; and the head' 

 and tail-folds of the amnion {hd-fl.amn. and tl-fl.amn.). The short oblique 

 canal perforating the embryonal area [emhr.ecto.) is the neureuteric canal, and 

 behind it is the primitive streak region, the mesoderm of which is directly 

 continued into the connecting stalk [est.), 



a point of considerable interest, to which I refer again later. 

 The cellular mass, so formed, extends downwards and back- 

 wiirds in contact with the inner surface of the. attaching area 

 of trophoblast, and in all but its proximal attached part it 



* Prof. T. H. Bryce, in 1908, was, I find, the first to put forward the view that this 

 mesoderm in Tarsius is to be considered as the equivalent of the mesoderm which 

 is proliferated from the posterior end of the pirimitive streak in lower Mammals. 



