FORMATION OP GERMINAL LAYERS IN OHELONIA. 23 



ill the ventral median line. They pass above gradually into the 

 loose network of cells which has now extended itself entirely 

 across. The meshes of the network have also become larger 

 than in the previous sections. It is evidently this loose network 

 that produced the appearance of a honeycomb in fig. 1 b. 



Fig. 14 passes in front of the ventral opening of the blasto- 

 poric passage, and indicates that the loosely scattered lower 

 layer cells are here arranging themselves into the chorda- 

 entoblast in the ventral median line of this region, i. e. along 

 the front part of the middle prong of the trident apparent in 

 the surface views (figs. 1 a and b). 



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

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

 entire entoblast 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 a, PI. Ill, 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 digestive 

 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. 



Fiff. 16 is thfi median longitudinal section of an embryo 

 taken from the same lot as that represented in fig. 1. The 

 blastoporic passage is very distinct. On its dorsal lip, the 

 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, the ectoblast, 



