FORMATION OF GERMINAL LAYERS IN CHELONIA. 2l5 



embryonic shield, but gradually thinning out to a single layer of flat 

 cells toward both sides. The yolk occupies the entire lower stratum. 

 Nuclei (//. n.) are visible in it. The space between the ectoblast and 

 the yolk is occupied by a mass of mesoblast cells which is here dis- 

 tinctly :?eparate from both the ectoblast and the yolk. 



In fig. 8 (which by the way is taken from another embryo of the 

 same deposit, as the section corresponding to this in the first series is 

 unfortunately injured) the ectoblast is continuous in the median line 

 with the mesoblast, i. e. it is very actively proliferating and giving 

 oft' cells abundantly to the mesoblast. 



Fig. 9 passes through the region directly behind the blastopore. 

 The ectoblast is distinct laterally, but toward the median line, and at 

 some distance from it, passes gradually into a mass of cells in Avhich 

 no layers can be distinguished. Difl:erent from fig. 8 where the 

 ectoblast cells, although continuous in the median line with the mass 

 below, still maintain their columnar shape over the whole dorsal 

 surface and thus give an impression of the ectoblast extending entirely 

 across, the ectoblast is in this section fused into the median mass 

 without retaining the slightest trace of the columnar arrangement, 

 and the median mass of cells thus expose their surface to the exterior 

 for a short space in the axial line (_yk. p.). We wish to emphasize the 

 fact that this part directly behind the blastopore is neither at this nor 

 at any suljsequent time until considerably later (if ever at all), covered 

 by the ectoblast of the general surface of the dody. This area we 

 consider to be the remnant of the yolk-plug of Rusconi 

 found in the Amphibian embryos. This will become clear in 

 the later stages. From the axial mass, where the layers are indis- 

 tinguishable, there extends toward each side a thick mesoblastic wing 

 under the ectoblast. The yolk seems to be distinct from the mass 

 above, although, throughout this region, protoplasmic threads seem to 



