26 K. MITSUKDia AND C. ISHIKAWA. 



along the median line in front of it, there is an opacity. This 

 seems to be due simply to the fact that the cell layers are 

 thicker in this region than elsewhere. In the middle of this 

 opacity in front of the blastopore and apparently starting from 

 the latter there is a shallow median groove, which probably 

 corresponds to the " Primitivrinne "" or " Riickenrinne " 

 described by Hertwig in the Triton embryo (No. 6). On the 

 ventral side (fig. 2 b), we wish to call especial attention to the 

 ventral opening of the blastoporic passage {v.o.). In the 

 previous stage, it was a circular opening without definite limits. 

 In this stage it has acquired well-defined limits on all sides ex- 

 cept towards the front, where it is only faintly bounded. Along 

 the median line of the roof of the recess thus formed, a wide 

 low ridge is visible and is continued in some specimens in 

 front of this area. This is undoubtedly the chorda-entoblast. 

 In the ventral view, the posterior part is concealed by 

 a mass of yolk which has accumulated here in the process of 

 removing the shield from the egg. 



As we are going to describe somewhat in detail the next 

 stage, we may omit the description of the sections of this, 

 except one through the ventral opening of the blastoporic 

 passage. Fig. 17 is such a section. It passes through the front 

 part of the lower opening. There is in the median line a 

 slight notch in the ectoblast which corresponds to the groove 

 seen in the surface view. In the entoblast we see the axial 

 chorda-entoblast formed as usual of columnar cells. Late- 

 rally, it passes on each side into a mass of polygonal cells — 

 the darm -entoblast of Hertwig (No. 6) — which becomes in 

 its turn continuous with the yolk at the edge of the em- 

 brvonic shield. At the point where the chorda-entoblast 

 and the darm-entoblast meet each other, the darm- entoblast 

 projects as a ridge into the digestive cavity and thus con- 

 stitutes one of the lateral edges which bound the ventral 

 opening of the blastoporic passage (compare fig. 2 b). Con- 

 forming to the groove in the ectoblast, the chorda-entoblast 

 projects downwards in the median line. This corresponds no 

 doubt to the ridge seen in the surface view within the lower 



