FORMATION OF GEBMINAL LAYERS IN CHELONIA. 221 



the horseshoe is the rudimentary yolk-plug. Round the blastopore, 

 and along the median line in front of it, there is an opacity. This 

 seems to be due simply to the fact that the cell layers are thicker in 

 this region than elsewhere. In the middle of this opacity in front of 

 the blastopore and apparently starting from the latter there is a shallow 

 median groove, which probably / corresponds to the "Primitivrinne" 

 or " Rückenrinne " described by Hertwig in the Triton embryo (No. 6). 

 On the ventral side (fig. 2 fc), we wish to call especial attention to the 

 ventral opening of the blastoporic passage (y. o.). In the previous 

 stage, it was a circular opening without definite limits. In this stage 

 it has acquired well-defined limits on all sides except towards the front, 

 where it is only faintly bounded. Along the median line of the roof 

 of the recess thus formed, a wide low^ ridge is visible and is continued 

 in some specimens in front of this area. This is undoubtedly the 

 chorda-entoblast. In the ventral view, the posterior part is concealed 

 by a mass of yolk which has accumulated here in the process of 

 removing the shield from the egg. 



As we are going to describe somewhat in detail the next stage, 

 we may omit the description of the sections of this, except one through 

 the ventral opening of the blastoporic passage. Eig. 17 is such a 

 section. It passes through the front part of the lower opening. 

 There is in the median line a slight notch in the ectoblast which cor- 

 responds to the groove seen in the surface view. In the entoblast we 

 see the axial chorda-entoblast formed as usual of columnar cells. 

 Laterally, it passes on each side into a mass of polygonal cells — the 

 darm-entoblast of Hertwig (No. 6) — which becomes in its turn con- 

 tinuous with the yolk at the edge of the embryonic shield. At the 

 point where the chorda-entoblast and the darm-entoblast meet each 

 other, the darm-entoblast projects as a ridge into the digestive cavity 

 and thus constitutes one of the lateral edo-es which bound the ventral 



