226 K. MITSUKUEI AND C. ISHIKAWA. 



sections after which tlie ectoblast l)öcomc8 independent. Tlie entohlast 

 seems, however, to be connected with the mesoblast for a greater 

 length and to be actively contributing cells to the latter. This is the 

 region where the germinnl wall makes a horseshoe-shaped bend round 

 the posterior part of the embryo (fig. 5). Except in this last detail, 

 the relations of the various parts behind the blastopore are exactly 

 as in the stage represented in fig. 1. 



Going forward, fig. 19 passes through the blastoporic passage. 

 As it is directly in front of the dorsal lip of the blastopore, the 

 ectoblast is still continuous for a little space with the cliorda-entoblast, 

 which as usual vaults over the passage. The columnar cells extend 

 to the sides also, but on the floor of the passage the cells are poly- 

 gonal, so that this part which is the continuation of the yolk-phig 

 differs in its appearance from the roof and the sides. To this pari, 

 too, the darm-entoblast (end.) is attached. From the entire side of 

 the axial mass the mesoblastic sheet goes out on each side. 



Fig. 20 passes slightly in front of the ventral opening of the 

 blastoporic passage. In the median line the chorda-entoblast ( enc.) 

 forms directly the roof of the digestive cavity, without the interven- 

 tion of the darm-entoblast ( end.) which stops at a short distance from 

 the axial line. On the left side of the section, more clearly than on 

 the right, the darm-entoblast is seen to make a fold at its innermost 

 point where it abuts against the chorda-entoblast and then to turn 

 outside again to be lost in the mesoblast. The mesoblast is therefore 

 partly continuous with the chorda-entoblast and partly with the darm- 

 entoblast. In other words, it starts from the point where the cliorda- 

 and the darm-entoblast meet each other. The mesoblast cells in this 

 region show a peculiar arrangement. Those cells next the ectoblast 

 are C(jlumnar and look like the continuation of the chorda-entoblast. 

 The cells placed ventrally to these are polygonal and without any 



