FORMATION OP GERMINAL LAYERS IN CHELONIA. 19 



yolk. The general appearance of the embryonic shield at this 

 stage is represented in fig. 1 a and h, enlarged about thirty 

 diameters. Fig. 1 a, shows it as seen from the dorsal side, 

 and fig. 1 h, as seen from the ventral side after the removal of 

 the shield from the egg. The embryonic shield does not lie in 

 the centre of the area pellucida {a. p.), but is placed excentri- 

 cally nearer its hind end, so that here it is continuous with the 

 area opaca {a. o.). Theectoblast has already spread itself over 

 a large part of the egg, although we did not determine its exact 

 limits (see fig. 16). On the dorsal view the blastopore [bl., 

 fig. 1 a) forms the most conspicuous feature ; it is seen as a 

 wide transverse slit across the posterior part of the embryonic 

 shield, occupying considerably more than one third of the 

 breadth across. From the blastopore a passage leads obliquely 

 forward and ventral ward, and opens about in the centre of the 

 ventral surface with a circular opening (v. o.). The walls of 

 the ventral opening are posteriorly quite high, but become 

 gradually lower and lower toward the front, until they sink to 

 the general level of the ventral surface. For the sake of 

 brevity this passage, leading from the blastopore dorsally and 

 opening below, we shall hereafter call the blastoporic passage. 

 It becomes eventually the neurenteric canal. Returnrhg to 

 the dorsal surface, the shield in front of the blastopore presents 

 a broad flat expanse, in which are seen indistinctly three opaque 

 lines radiating from behind forward, like the prongs of a 

 trident. On referring to the ventral side we see that the two 

 lateral opaque lines correspond to the thickenings which form 

 the walls of the inferior opening of the blastoporic passage. 

 Accordingly they are thickest posteriorly, and gradually thin 

 out toward the front. The middle prong of the trident cor- 

 responds to the roof of the blastoporic passage, and its con- 

 tinuation to the front edge of the embryonic shield. It is, in 

 fact, the chorda entoblast, which is still in the process of 

 formation in front, as will be made clear by sections. The 

 remaining parts of the ventral surface not taken up by these 

 three thickenings present the appearance of a honeycomb. 

 Of this we shall speak later on. Coming back to the dorsal 



