lO Primitive Streak and Notochordal Canal in Chelonia. 



in Figure 32, and that its anterior end approaches the roof of the 

 canal at the point marked by an asterisk. The cells forming the 

 roof of the canal somewhat in front of this point have lost in great 

 part their columnar arrangement, and have taken on that vacuo- 

 lated condition seen in the region of Figure 32 marked by a dag- 

 ger. The central area of the entoderm beneath the shield is not 

 only vacuolated, but a ventral layer seems to be splitting off from it. 

 If the cavity of this central area is continuous with the notochordal 

 invagination, and I believe that it is, then the lumen of the latter 

 extends forward a distance equal to two thirds the length of the 

 shield. In the next section. Figure 34, the posterior solid ventral re- 

 gion has become completely fused with the anterior ventral strand. 

 At their point of fusion, however, they are united for the space of 

 two cells with the dorsal wall of the canal. From this series of 

 sections one is led to believe that the lumen of the blastoporic 

 canal had not attained its definite lateral extent at the time when 

 it first opened towards the yolk. In the lateral regions the lumen 

 of the notochordal canal cannot be considered to extend forward 

 for more than two thirds the length of the shield. Returning now 

 to the median section (Fig. 32), we find that the columnar arrange- 

 ment of the cells forming the roof of the canal has extended for- 

 ward over about two thirds of the length of the shield ; it seems 

 probable, therefore, that the anterior end of the columnar region 

 was the anterior limit of the canal at the time its floor broke 

 through. It is possible that cells from the primitive knob have 

 gone on proliferating to form the vacuolated entoderm of the 

 remainder of the anterior part of the shield. The extent of the 

 canal at the time it breaks through seems to indicate that the 

 embryos correspond with the stages described by Mehnert ('92, p. 

 411) and Mitsukuri ('93, p. 241) rather than with those of Will's 

 tortoise ('92*, pp. 191, 192). Will says: " Aus diesen Stadien 

 geht nun die wichtige Thatsache hervor, dass auch der Urdarm 

 der Schildkrote noch in seiner ganzen Ausdehnung hohl ist und 

 dass seine Ausdehnung absolut und relativ diejenige des Gecko 

 noch iibertrifft. Wahrend derselbe beim Gecko die vorderen und 

 seitlichen Rander des Schildes nie vollstandig erreicht, nimmt 

 derselbe bei der Schildkrote stets die ganze Flache des Schildes 

 em. 



Sections through the anterior unbroken area of the shields 

 of embryos of Chrysemys picta (Plate I. Fig. i') and Ozotheca 



