46 K. MTTSUKURI. 



soon increases in bulk and spreads out behind in all directions like an 

 open fan. From the two edges of this fan, the formation of the gastral 

 mesoblast proceeds gradually forward (see Fig. 2 and its sections Figs. 

 13-22). It has sufficiently been sliown nbove th;it a certain stretcli of 

 epithelium found on each side of the chordn-anlage in enrly embryos 

 (like that given in Figs. 1 and la) l3ecomes eventually a portion of the 

 mesoblast. Only there is as yet nothing to distinguish it as such. 

 While the remaining part (i. e. the gut-hypoblast) of the lower layer 

 becomes gradually reduced to a single stratum of columnar cells, this 

 stretch (^f epithelium distinguishes itself by proliferating cells whicli 

 spread |)artlv upwards, but mostly outwards Ijetween the epiblast and 

 the gut-hvpoblast (the embryo given in Fig. 2 and its sections Figs. 

 13-22). At this stage, therefore, the internal end of the gut-hypo- 

 blast on each side is at a considerable distance from the chovdii-anlage, 

 (Figs. 12 and 16). The gut-hypo])last, hoAvever, soon extends itself 

 towards the chorda- anläge, shutting otf the epithelial stretch in question 

 from the enteric cavity (Fig. 17). This inward extension of the gut- 

 hypoblast is ])i'obably the cause of the grooves converging posteriorly 

 into the single median chorda groove seen in the surface views. Figs. 2 

 and Sa. Tliis movement seems also to be the signal for the se])aration 

 of the three structures : the Ghordii- anläge, the mesoblast, and the de- 

 finitive or gut-hypoblast (Fig. 18). The separation is, however, com- 

 plete only in the middle of the embryo, for, as we go backward and 

 approach the blastoporic passage, the three structures are again united 

 (Fig. 19 and 20). This is necessarily the case, I think, because the 

 posterior part of the embryo is growing in length. In Fig. 22, we 

 see that the yolk-plug has become very distinct. This has been 

 brought about by the change of shape in the dorsal opening of the 

 blastoporic passage. Whereas it is at first concave forwards as in 

 Fig. 1, it becomes later horse-shoe shaped (see Figs. 3 and 4), with 



