32 K. MITSUKQRI AND C. ISHIKAWA. 



is absent^ and thus no additions are made laterally to the meso- 

 blast from the entoblast. 



Returning to the middle region of the body, figs. 21 and 22 

 serve to show the first steps in the formation of the notochord. 

 The chorda-entoblast which in fig. 20 passed laterally without 

 any interruption into the mesoblast, is in fig. 21 marked off 

 from the mesoblast, at least in the upper part. The cells at the 

 border between the two are turned away from one another ; 

 thus the cells of the chorda are directed inwards and down- 

 wards, while the contiguous cells of the mesoblast are directed 

 outwards and downwards. The mesoblast is still united with 

 the darm-entoblast. As yet, the chorda is only a 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 dis- 

 tinctly 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 fig. 16. Its angle 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 blas- 

 topore the ectoblast is reflected and becomes continuous with 

 the chorda-entoblast. 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.) 



