STUDIES IN MAMMALIAN EMBETOLOGY. 515 



Still I must now introduce a new factor into our considera- 

 tions, viz. that not all the mesoblast-cells enclosed -within the 

 circle here indicated have arisen from the proliferation along 

 the gastrula ridge just described, but that many of them can 

 be seen to take their origin from what I have termed the third 

 source from which mesoblast arises, viz. the annular ring of 

 hypoblast round and outside the edge of the embryonic shield 

 (figs. 33 — 35, 53 — 55,hy.az.). Figures are given on PI. XXXIX 

 (figs. 57 — 61) in which the participation of this hypoblastic 

 zone towards the formation of mesoblast is put beyond all 

 doubt. They are taken from three different embryos, and in 

 all of them the plane according to which the nuclei divide, as 

 indicated by the well-preserved karyolytic figures, is such as 

 to leave no doubt that in each of these cases a cell divisioh 

 was just on the point of taking place, one of the cells of which 

 was going to number amongst the mesoblast-cells, whereas 

 the other was going to remain in the underlying layer of 

 hypoblast. 



That, moreover, this latter layer also increases by multiplica- 

 tion of its cells can be demonstrated in these same prepara- 

 tions ; for such an increase the plane of the nuclear division is, 

 however, seen to be not coincident with that of the hypoblast, 

 but, on the contrary, perpendicular to it. 



Fig. 58 gives the most strongly marked karyolytic stage ; 

 fig. 59, X, a stage in which the nuclei have just regenerated 

 after division. This latter figure would ofi'er no convincing 

 proof of the process here advocated ; it is, however, a welcome 

 corroboration of what is noticed in earlier phases in the other 

 figures. The reconstructions on PI. XXXVIII allow us to point 

 at the exact spots, when seen from above, where the karyolytic 

 stages of the foregoing figures are situated. We can then see 

 that it is not only the posterior, but also the lateral parts of 

 the annular ring that thus contribute to the formation of 

 mesoblast-cells. 



Also in the anterior portions of the hypoblastic ring, the 

 participation of its constituent cells towards the formation of 

 mesoblast can be proved by karyolytic figures. It would, 



VOL. XXXI, PAKT IV. — NEW SEE. M M 



