228 



K. MITSUKUEI AND C. ISHIKAWA. 



mass of columnar cells. In fig. 22, five sections in front of fig. 21, 

 the chorda has become rounded in outline and considerably smaller in 

 section. The most dorsal and median cells alone are columnar, and 

 the remaining cells are arranged as if the more lateral cells have folded 

 inwards and downwards from the two sides and met in the median 

 line. The mesoblast is now distinctly separated from both the chorda 

 and the darm-entoblast. The last abuts against the chorda, but seems 

 separate from it. This is as far as the formation of the chorda has 

 advanced in this stage. In front" of fig. 22 the chorda becomes wider 

 again, until in the region of the head-fold it is as represented in 

 fig. 23 ; a similar arrangement is found at the posterior end of the 

 embryo. 



Fig. 24 is a longitudinal section of another embryo from the 

 same deposit of eggs as the one represented in fig. 3. It passes very 

 nearly in the median line. The blastoporic passage is considerably 

 narrower than in fio;. 16. Its an o-le of inclination to the surface of 

 the ectoblast is now greater, approaching more nearly a right angle ; 

 hence it has become also much shorter than before. At the dorsal lip 

 of the blastopore the ectoblast is reflected and becomes continuous 

 with the chorda-en toblast. Owing to the fact that the chorda is most 

 developed and therefore narrower in the middle region of the embryo 

 than in front or behind, and perhaps also to the fact that the section 

 is slightly oblique, the mesoblast (mes.) appears for a short space 

 (* — c.) in this section. The entoblast, which is very thick in front, 

 especially in the head-fold [It. f. marked by a notch in the ectoblast), 

 becomes suddenly reduced at the point c into a thin ventral layer 

 (end.) which stretches posteriorly as far as the point marked with a * , 

 where it seems to unite with tlie chorda-entoblast. From the point of 

 junction and also continued forward from the chorda-entoblast, the 

 mesoblast sheet stretches forwards as far as c, above the darm-ento- 



