M :\ 1:1 OPMEK ; OF i in: TWO i'liiMAHY (;I:KM i A 



It 5 



( >p ciallv In-hind (/*/) t h- ci e, five nuel. i Mtt constantly 



to be found k.- piu; up tin- suppl.-nn-ntary cleavage. 



Owiuir t tin- ajip aianre of tin- new ti>suiv ( subirrriiiinal cai 



(fig. 53 ?>'/), the cleavage-cavity (li;_ r . f> 1 /'//) is almost completely 



obliterati d. The two cell-layers of the 



Mastula -stair* 1 (fig. 51 dw, vw), described as 



Ivinir our aliovr and one below the ch ::VM; > - 



raviiv, have come close together (figs. 53 



and 51), ht in.ij s parated from each otln-r 



l.y only a narrow fissure. In the upp r 



lavt r ("/) t lie cells havr assunu-d a cubical, 



and at a somewhat later stage a cylindrical. 



form, and constitute a compact epithelial 



int inbrane. The lower layer (ik) is composed 



of larger roundish and loosely arranged cells 



in several layers. The former is the primary 



out. r p rm -layer, the latter the inner layer. 



In the region of the posterior marginal 



i idirc (vl), where the cells are at the same 



tiint- engaged in more active proliferation, 



the two layers are continuous with each 



other. 



The highly important processes, by means 

 of which are produced the conditions repre- 

 sented in figs. 53 and 54, present many points 

 of comparison with the gastrulation of the 

 Selachians and Amphibia. We can conceive 

 that the nrwlv appearing fissure has arisen, 

 as in the case of the p-rin-disc of Pristiurus 

 (fig. 50), by an infolding, in Mich a way that, 

 as in the former case, cells grow inward from 

 the posterior maririisal rid^-: and that at 

 the same time, at the deep part of the in- 

 vairination. the cells Mhich are originally 

 continuous with the yolk (fig. 53 dk) 1 -tat-h 

 thrmsdvis from the latter, and are employed for the increase of the 

 inner :< nu layer. 



If this explanation is correct, the fissure (ad) which now exists be- 

 twccn the inner germ-layer and the floor of the yolk corresponds to 

 the co?lenteron, as GOETTE and EAUBKH have already remarked, and 

 as DUVAL has for the first time demonstrated ; moreover, the cres- 



