220 



K. MITSUKUKI AND C. ISHIKAWA. 



fctoblast and the entoblast are fused, and from the point of fusion tlie 

 mesohlast is being given off posteriorly in all directions for the space 

 of 180°. 



6. Also, behind the place where the two primary layers are thus 

 fused, the ectoblast is giving off cells to the mesoblast along the 

 median line (fig. 8). This is the line of the primitive streak. It is 

 very short and is present in only two or three sections, 



7. The mesoblastic mass derived from the two sources mentioned 

 in (5) and (6) is unpaired and constitutes the transverse swelling in 

 the posterior part of the embryonic shield, " the sickle." This is the 

 only place where the mesoblast is present at this stage. 



8. The median mass formed by the fusion of the three layers at 

 the posterior wall of the blastoporic passage appears for a short space 

 on the dorsal surface (fig. 9) — the remnant of the yolk-pl ng of 

 Rusconi. 



Formation of the Mesoblast and of the Chorda Dorsalis. 



In the previous stage, the mesoblast was found only in the region 

 behind the blastopore. We may now proceed to describe its formation 

 in front of the blastopore. We call attention first to the embryo 

 represented in figs. 2 a and h. It was taken out exactly forty-eigh 

 hours after the deposition, but as the weather was unusually cold for 

 the season during the interval, it has made very little progress in 

 developnient, and is not as far advanced as many thirty-six hours old. 

 As before, a dorsal and a ventral view of the embryonic shield is given , 

 although these are not taken in this case from the same embryo. The 

 shape of the shield has not changed materially from the previous stage. 

 In the dorsal view (fig. 2r<) the blastopore has assumed a horseshoe 

 shape, and is more of a slit than bef )re. Occupying the concavity of 



