THE EARLY DEVELOPMEXT OF THE MAESUPIALFA. 49' 



to their own territory, so that we are justified in regarding 

 the vesicle wall as composed of two independently growing 

 zones. Now the existence of two such independent zones in 

 the unilaminar wall is, to my mind, only intelligible on the 

 view that they are the products of two originally distinct, 

 predetermined cell-groups, and if this be admitted, then I 

 think we are justified in concluding, in view of the facts 

 already set forth, that the two zones in question are derived, 

 the one from the upper cell-ring of the 16-celled stage, the 

 other from the lower ring ; that, in other words, they repre- 

 sent respectively the upper and lower hemispheres of the 

 early blastocysts. 



If, now, we find that the embryonal ectoderm and the ento- 

 derm arise from one of these two regions of the unilaminar 

 wall, Avhilst the other directly forms the outer extra-embryonal 

 layer of the later (bilaminar) vesicle, then we must designate 

 the former region as the upper or formative, and the latter as 

 the lower or non-formative. Further, bearing in mind the 

 characters of the cells of the two rings of the 16-celled stao-e, 

 I think we are justified in holding that the formative region 

 is derived from the ring of smaller, less yolk-rich cells, and 

 the non-formative region from the ring of larg-er, more yolk- 

 rich cells, even if it is impossible to demonstrate an actual 

 genetic continuity between the constituent cells of these two 

 rings and those forming the independently growing- areas of 

 the later blastocyst. I have recently re-examined a series of 

 vesicles, measuring 1*5-1 "8 mm. in diameter, obtained from a 

 female killed in 1906, and I have so far found it impossible, 

 either in the entire vesicle or in portions of the wall stained 

 and mounted on the flat, to distinguish between the cells over 

 opposite hemispheres. Thus the only actual guide we have 

 for the determination of the poles in such vesicles is the 

 yolk-body, and though the latter is liable to displacement, it 

 is worthy of record that I have several times found it in 

 relation to the formative area in vesicles 4'o-6 mm. in 

 diameter, but never in relation to the non-formative region. 

 This evidence is, therefore, so far as it goes, confirmatory of 



VOL. 56, PART 1. — NEW SERIES. 4 



