FOKMATION OF GEEMINAL LAYERS IN CHELONIA. 213 



to obtain still earlier stages by opening pregnant females have proved 

 but partially successful. In almost every case, with only some doubt- 

 ful exceptions, the eggs we found in the oviduct were unfortunately 

 fully as much advanced as those just laid. 



On opening an egg directly after its deposition the blastoderm is 

 always found at the pole turned above. The embryonic shield, with 

 the pellucid area around it, stands out conspicuously as a small, nearly 

 circular spot, on the yellow surface of the yolk. The general appear- 

 ance of the embryonic shield at this stage is represented in figs. 1 a and 

 h, enlarged about thirty diameters. Fig. 1 a, shows it as seen from 

 the dorsal side, and fig. 1 6, 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 (ctp.), but is placed excentrically 

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

 (ao.). The ectoblast nas already spread itself over a large part of the 

 egg, although we did not determine its exact limits (see fig. IG). On 

 the dorsal view the blastopore (W., fig. 1 ft) 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 ventralward, and opens about in the centre of the ventral 

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

 ing 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 blasto- 

 pore dorsally and opening below, we shall hereafter call the blastoporic 

 passage. It becomes eventually the neurenteric canal. Returning to 

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

 fiat ex])anse, in which are seen indistinctly three opaque lines radiating 

 from behind forward, like the prongs of a trident. On referring to the 



