30 K. MITSUKURI AND C. ISHIKAWA. 



median fissure. What appears to be such is the optical ex- 

 pression of the primitive streak, along which the ectoblast is 

 proliferating, and giving cells to the mesoblast below. Even 

 in the earliest embryos with this appearance it is doubtful if 

 it ever extends to the extreme tip of the plug. As far as we 

 are aware, the only author who mentions what seems to be the 

 yolk-plug in an amniotic Vertebrate is Gasser, who observed 

 it in an abnormal fowl embryo (No. 4, Taf. x, figs. 4 — 7). The 

 reason why the yolk-plug in Trionyx is more conspicuous at 

 this stage than earlier stands, we think, in close connection 

 with the fact that the blastopore has become a much better 

 defined horseshoe-shaped slit. 



We return now from this long digression to the description 

 of the embryo before us. The sections behind fig. 18 show 

 that immediately behind the yolk-plug, which persists distinctly 

 in only one more section after fig. 18, the ectoblast extends 

 over the whole surface as shown by the characteristic columnar 

 cells. For a short space, however, the ectoblast is proliferating 

 in the median line and is continuous with the mesoblast below. 

 This is seen in only three sections after which the ectoblast 

 becomes independent. The entoblast seems, however, to be 

 connected with the mesoblast for a greater length and to be 

 actively contributing cells to the latter. This is the region 

 where the germinal wall makes a horseshoe-shaped bend round 

 the posterior part of the embryo (fig. 5). Except in this last 

 detail, the relations of the various parts behind the blastopore 

 exactly as in the stage represented in fig. 1. 



Going forward, fig. 19 passes through the blastoporic passage. 

 As it is directly in front of the dorsal lip of the blastopore, the 

 ectoblast is still continuous for a little space with the chorda- 

 entoblast, which as usual vaults over the passage. The columnar 

 cells extend to the sides also, but on the floor of the passage 

 the cells are polygonal, so that this part which is the continua- 

 tion of the yolk-plug dift'ers in its appearance from the roof 

 and the sides. To this part, too, the darm-entoblast {end.) is 

 attached. From the entire side of the axial mass the meso- 

 blastic sheet goes out on each side. 



