280 K. MITSUKUEI AND C. ISHIKAWA. 



S .^p-ira ted fvon\ i 1 1 )y an interval av li e r e cells are most active- 

 ly 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-entohlast, 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 meso- 

 blastic masses have separated from the chorda- and darm-entoblast. 

 The chorda-entoblast is arcuate. The darm-entoblast abuts against 

 it Init is distinctly separate from it. In fig. 27, the third section 

 behind fig. 26, the chorda-entoblrst has become a cord-like mass, 

 against the more ventral side of which the darm-entoblast of both 

 sides is applied. This corresponds to fig. 22 of the previous stage. 

 In fig. 28, the third section behind fig. 27, the darm-entoblast has 

 passed imder the notochord from both sides, and united so as to 

 form a continuous sheet across. The formation of the notochord is 

 thus completed. 



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 passage, 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« and h, are introduced to 

 illustrate this process. 



Fig. 30 is the most posterior section given. It is slightly in 



