218 K. MITSUKUßl AND C. ISHIKAWA. 



median line of this region, i. e. along tlie front part of the middle 

 prong of the trident apparent in the snrface views (figs. 1 a and h). 



Fig. 15 passes near the front end of the embryonic shield. 

 There is no longer any trace of the chorda-entoblast; the entire 

 en toblast is an irregular stratum of stellate cells not thick enough to 

 form a network. It passes into the yolk at the sides. 



We may here call attention to the appearances which are seen in 

 some embryos of this stage. Round the edge of the lower opening of 

 the blastoporic passage, especially toward the front, there is a shelf- 

 like extension of the entoblast into the archenteric cavity somewhat 

 like the velum of a hydromedusa. Fig. 14^, 1*1. XVI, represents such 

 an appearance. The section is well in front, so that the shelf-like 

 extension is continuous across and divides a small space above from 

 the main digesti^^e cavity below. In sections posterior to this, the 

 small space opens below. We do not know what the significance of 

 this is, unless we suppose that the embryo is younger than that given 

 in fig. 1, and therefore the ventral opening of the blastoporic passage 

 is not yet entirely clear. 



Fig. 16 is the median longitudinal section of an embryo taken 

 from the same lot as tliat represented in fig. 1. The blastoporic 

 passage is very distinct. On its dorsal lip, tlie ectoblast is reflected 

 forwards and downwards and becomes continuous with the chorda- 

 entoblast which passes in front into a loose network of cells with wide 

 meshes, and finally, into the yolk at the edge of the embryonic shield. 

 At the posterior wall of the blastoporic passage, the three layers, tlie 

 ectoblast, the mesoblast, and the yolk (i. e. the entoblast) are merged 

 into one another; in other words, the ectoblast and the entoblast are here 

 fused and from the fused place a mass of mesoblast cells extends pos- 

 teriorly. The three layers become independent a short distance behind 

 the blastopore. As the cross-sections of this region (figs. <S, 9, and 



