FORMATION OF GERMINAL LAYERS IN CHELONIA. 



219 



10) show that the mesoblastic mass is similarly extending to each side, 

 we may conclude that, in addition to the primitive streak (fig. 8), the 

 mesoblast is being given off from the posterior wall of tlie blastoporic 

 passage or at least from its upper part, in all postérieur directions for 

 an arc of 180°, somewhat in the shape of an open fan; and this 

 posterior unpaired mesoblastic mass causes the swelling known as the 

 " sickle." Examining the ectoblast of the posterior part more in 

 detail, we find it gradually losing its colu.nnar character as we 

 approach the blastopore from behind, but the space where the fused 

 median mass of cells is dorsally exposed to the exterior, viz. the yolk- 

 plug (compare fig. 9) is not as conspicuous in the longitudinal section 

 as in the later stages. The entoblastic part of this fused mass extends 

 quite forward. This corresponds to the cells seen in the floor of the 

 blastoporic passage in fig. 11. A slight projection from its extreme 

 tip is, we imagine, the remnant of the shelf-like structure mentioned 

 in reference to fig. 14 a. 



The principal facts brought out by the study of this stage may 

 be summed up as follows : 



1. There is a passage which, beginning with the blastopore on 

 the posterior part of the dorsal surfxce, takes a forward and downward 

 course to the ventral surface, opening in about the middle part of the 

 latter by a circular opening. 



2. At the dorsal lip of the blastopore the ecfoblast is reflected 

 and becomes continuous with the chorda-entoblast. 



3. In front of the blastopore there are as yet only two prhnary 

 layers, the ectoblast and the entoblast. 



4. The entoblast is having its axial part arranged into a columnar 

 epithelium to form the chorda-entoblast. This process proceeds from 

 behind forward. 



5. At the posterior wall (i. e. floor) of the blastoporic passage the 



