22 Primitive Streak and Notochordal Canal in Chelonia. 



and four KoUer ('82, pp. 194, 195) finds no sharp boundary 

 between the cells of the germ wall and the crescent. A decrease 

 in the size of the crescent is accompanied by an increase in 

 the length of the streak. But to Duval ('78, '84, '89) is due 

 the credit of first applying from observation the theory of 

 concrescence to birds. 



I ' When we come to reptiles the evidence that the streak takes 

 part in the formation of the axis of the embryo is meagre. Strahl 

 ('82) states that the notochordal invagination arises within the 

 streak, and that portions of the streak anterior to the invagination 

 are rapidly converted into the axis of the embryo. Moreover, 

 he asserts that the neurenteric canal travels backward along the 

 streak. According to Will the primary neurenteric canal in the 

 gecko, and possibly in the turtle, closes to open later in a posterior 

 region of the streak. Kupffer ('82, Taf IV. Fig. 40, e, f) has 

 figured for Coluber a short posterior axial continuation of the blas- 

 toporic opening. "In reptiles, then," writes Minot ('92, p. 123), 

 " concrescence can only be inferred from the presence of the 

 ' Sichel ' and the growth backward of the primitive axis." 



Despite a lack of corroborative evidence of concrescence in 

 reptiles, I consider the present case one of true spina bifida. The 

 decided evidence of continuity between ectoderm and entoderm in 

 one half of one anterior section (Plate V. Fig. 25) justifies me, I 

 believe, in concluding that the primitive groove extended forward, 

 at least to this point. Perhaps the failure of the ectodermal halves 

 of the blastoporic rim to fuse may account also for the failure in 

 the development of the normal notochordal canal (compare Roux, 

 '88, p. 143). 



In regard to embryo /3 of this set of twins, little need be said. 

 The remaining portion of embryo /3 was cut, as stated, into sagittal 

 sections. A section which passes through the blastopore of this 

 embryo in the sagittal plane is represented in Plate VI. Fig. 28. 

 Posterior to the ventral lip of the open blastopore a primitive 

 streak is well developed. The ventral opening of the notochordal 

 canal does not extend so far posteriad as to be included in this 

 section. The section next figured (Fig. 29) passes through the 

 left side of /3 lateral to the blastopore, and includes the region of 

 fusion between embryos a and /8. The point where this fusion 

 takes place will be recognized by the ectodermal fold. The cavity 

 of the notochordal canal of /8 lies between the layers marked 



