34 K. MITSUKUEI AND C. ISHIKAWA. 



dorsalis, Figs. 25 — 28 are selected to illustrate these 

 points. 



Fig. 25 is the most anterior section represented. It goes 

 through the posterior part of the head. The amnion is closed 

 over it, but the digestive cavity is still widely open below. The 

 darm-entoblast formed by columnar cells does not reach the 

 chord a-entoblast, but is separated from it by an interval 

 where cells are most actively proliferating and 

 giving rise to the mesoblastic mass. Fig. 29 is a 

 similar section from another embryo of the same stage. Here 

 also the chorda-entoblast, instead of passing directly into the 

 darm-entoblast, is separated from it on each side by a space 

 where cells are actively dividing and giving rise to the meso- 

 blast. This figure shows also more naturally than fig. 25 that 

 the mesoblastic mass consists of spindle-shaped and stellate 

 cells arranged in such a way as to give an impression of having 

 radiated from their origin. 



Figs. 26 — 28 show clearly the mode of the formation of the 

 notochord. Fig. 26 is two sections behind fig. 25. The 

 mesoblastic masses have separated from the chorda- and darm- 

 entoblast. The chorda-entoblast is arcuate. The darm-ento- 

 blast abuts against it but is distinctly separate from it. In 

 fig. 27, the third section behind fig. 26, the chorda-entoblast 

 has become a cord-like mass, against the more ventral side of 

 Avhich the darm-entoblast of both sides is applied. This cor- 

 responds to fig. 22 of the previous stage. In fig. 28, the third 

 section behind fig. 27, the darm-entoblast has passed under the 

 notochord from both sides, and united so as to form a continuous 

 sheet across. The formation of the notochord is thus com- 

 pleted. 



As in the previous stage, the notochord is finished only in 

 the middle region of the embryo. Toward the posterior 

 region, in front of the ventral opening of the blastoporic pas- 

 sage, the chorda is in the process of formation. The mode of 

 formation is exactly as at the front end. Figs. 30 — 34 from 

 an embryo of nearly the same stage as that represented in figs. 

 4 a and h, are introduced to illustrate this process. 



