24 K. MITSUKURI AND 0. ISHIKAWA. 



the mesoblast, and the yolk (i.e. the entoblast) are merged 

 into one another ; in other words, the ectoblast and the ento- 

 blast are here fused and from the fused place a mass of 

 mesoblast cells extends posteriorly. The three layers are inde- 

 pendent a short distance behind the blastopore. As the cross- 

 sections of this region (figs. 8, 9, and 10) show that the 

 mesoblastic mass is similarly extending to each side, we may 

 conclude that, in addition to the primitive streak (fig. 8), the 

 mesoblast is being given off from the posterior wall of the 

 blastoporic passage, or at least from its upper part in all 

 posterior directions for an arc of 180°, somewhat in the shape 

 of an open fan ; and this posterior unpaired mesoblastic mass 

 causes the swelling known as the " sickle.^' Examining the 

 ectoblast of the posterior part more in detail, we find it 

 gradually losing its columnar character as we approach the 

 blastopore from behind, but the space where the fused median 

 mass of cells is dorsally exposed to the exterior, viz. the yolk- 

 plug (compare fig. 9) is not as conspicuous in the longitudinal 

 section as in the later stages. The entoblastic part of this 

 fused mass extends quite forward. This corresponds to the 

 cells seen in the floor of the blastoporic passage in fig. 11. A 

 slight projection from its extreme tip is, we imagine, the 

 remnant of the shelf-like structure mentioned in reference to 

 fig. 14 a. 



The principal facts brought out by the study of this stage 

 may be summed up as follows : 



1 . There is a passage which, beginning with the blastopore 

 on the posterior part of the dorsal surface, takes a forward 

 and downward course to the ventral surface, opening in about 

 the middle part of the latter by a circular opening. 



2. At the dorsal lip of the blastopore the ectoblast is re 

 fleeted and becomes continuous with the chorda-entoblast. 



3. In front of the blastopore there are as yet only two 

 primary layers, the ectoblast and the entoblast. 



4. The entoblast is having its axial part arranged into a 

 columnar epithelium to form the chorda-entoblast. This 

 process proceeds from behind forward. 



