23 K, MITSUKURI AND C. ISHIKAWA. 



now irregularly circular in section. The roof and the sides of 

 the passage are formed by a columnar epithelium two or three 

 cells thick [enc). This is continuous with the ectoblast at the 

 dorsal lip of the blastopore (compare figs. 16 and 10). It is 

 the chorda-entoblast of Hertwig (No. 6). The floor of the 

 passage and the lower part in general is made up of irregularly 

 scattered cells. This is not only the continuation of the yolk- 

 plug but also of the yolk itself, which occupied the lowest 

 stratum in figs. 7 — 10, and which has been in the last two or 

 three sections gradually merging itself into the floor of the 

 blastoporic passage. Thus^ although it does not appear in any 

 single transverse section, the three germinal layers are fused 

 in the region behind the blastopore. Laterally the entoblast 

 is very thin and passes gradually into the yolk. The section 

 is out of the region of the " sickle/^ and there is no longer a 

 mesoblastic wing on each side. 



Fig. 12 passes through the posterior part of the lower open- 

 ing of the blastoporic passage. The thickenings which form 

 the lateral walls of the opening are therefore still quite thick 

 (compare fig. 1 b). The columnar chorda-entoblast is found 

 as before forming the roof and the sides of the passage, which 

 is now open below. Towards the lower part of the side walls 

 the columnar arrangement is lost and the cells are irregularly 

 scattered. Further out at the sides the cells form a loose 

 network, and then at the edge of the embryonic shield passes 

 into the yolk. 



Fig. 13 passes through the anterior part of the ventral open- 

 ing of the blastoporic passage, which has now flattened itself 

 out into a shallow groove in the median line. Its roof is still 

 formed by the distinctly columnar chorda-entoblast. Later- 

 ally, the chorda-entoblast gradually passes into a mass of cells 

 arranged in an irregular loose network, which in its turn is 

 replaced by the yolk at the edge of the embryonic shield. 



Passing forward, the chorda-entoblast begins gradually to con- 

 fine itself more and more to the ventral median surface, until in 

 the seventh section from fig. 13 it has the appearance^presented 

 iu fig. 14. Here the columnar shape is confined to a few cells 



