1 8 Primitive Streak and Notochordal Canal in Chelonia. 



to investigate the continuity of the primitive and medullary grooves 

 of individual a, we will direct our attention to sections of these 

 two individuals. Individual a was laid into transverse sections as 

 far as the right horn of the blastoporic slit in /8. The remaining 

 portion of /8 was then cut into sagittal sections. 



Beginning at the hind end of individual a, the primitive streak 

 and groove appear in two sections before the condition seen in 

 section three (Plate VI. Fig. 30) is reached. Free thickened ento- 

 derm, however, occurs in two sections posterior to the streak or 

 region of fused layers. In section three (Fig. 30) there is a sud- 

 den and decided deepening of the primitive groove, — a deepening 

 which seems to pass through the ectoderm~and end blindly in the 

 midst of the entoderm below. It is impossible to know how much 

 of this groove lies within ectoderm and how much within ento- 

 derm, for it is as difficult for me to determine how much of this 

 thickened area in which the groove lies owes its origin to ectoderm 

 and how much to entoderm, as it was for Selenka ('87, p. 116) to 

 determine the limits of ectoderm and entoderm in the region of 

 the primitive blastopore described by him for the opossum. This 

 fusion of layers with accompanying thickening in the fused area 

 extends through six sections, all of which show a primitive groove, 

 which displays this decided deepening in the third (the one 

 figured) and fourth sections counting from the posterior end of 

 the fused area. This deepening of the groove seems to be effected 

 by a separation and a disintegration of cells. The opening has 

 the form of a wedge which terminates below in a mere fissure. 

 The cells which border this opening show no indication of colum- 

 nar arrangement, but one or two of them show a tendency to 

 degenerate or become cornified. The fusion of ectoderm and 

 entoderm continues for the space of two sections anterior to the 

 region in which a lumen has appeared. The accumulation of cells 

 accompanying this fusion is, however, somewhat less, while the 

 groove suddenly becomes shallow again. The third and fourth of 

 the sections which possess the deep groove (Fig. 30), I consider to 

 include either the beginning of a developing neurenteric canal or 

 else a dwarfed canal. 



This restricted extent of the blastoporic opening is comparable 

 to the condition figured by Weldon ('83, Figs. 6 and 7) for Lacerta 

 muralis. Upon comparing Figure 30 with transverse sections of 

 the blastoporic opening of other Testudinidae, it will be seen that 



