FORMATION OF GEEMINAL LAYERS IN CHELONIA. 237 



cells wliicli acquire the columnar character from this point forward in 

 the median line of the future embryo, and arranged themselves into 

 the chorda-entoblast. This process has proceeded to the front end of 

 the embryonic shield in the embryo represented in tig. 1. Whether 

 there is any actual invagination of cells from the dorsal lip of the 

 blastopore we cannot tell, but this is of no moment so long as the 

 ectoblast becomes continuous with the axial strip of the entoblast 

 at the dorsal lip, and the arrangement of the lower layer cells into 

 the chorda-entoblast proceeds from here towards the front. We can 

 conceive the blastoporic passage itself arising in this way. As the cells 

 arrange themselves into the chorda-entoblast, these columnar cells 

 separate from the cells directly behind them and thus a fissure or 

 canal is produced just at the same rate as the cells arrange themselves 

 into the chorda-entoblast. The posterior wall of this canal would 

 thus be composed of undifferentiated cells, as it actually is. 



While the differentiation of the ectoblast thus stops, in the 

 median line, at the dorsal lip of the blastopore, and the above- 

 mentioned changes leading to the formation of the blastoporic passage 

 are going on, we can suppose that the differentiation of the ectoblast 

 is at the same time proceeding actively in the more lateral parts and 

 is extending backwards and meeting in the median line again slightly 

 behind the blastopore (see fig. 6). There would thus be left behind 

 the blastopore a small space not covered by the ectoblast. This is 

 the yolk-plug, which is of course continuous with the undifferentiated 

 cells forming the posterior wall of the blastoporic passage. From 

 the ectoblast in the median line behind the yolk-plug, cells begin to 

 proliferate and constitute the primitive streak. This may happen 

 before the blastoporic passage is completed (see Strahl, No. 8, Taf. 

 xiv, fig. 11). Proliferation begins also from the posterior wall of 

 the blastoporic passage. We shall then have a stage exactly like 



