Primitive Streak and Dorsal NotocJiordal Opening. 31 



a mechanical cause which could have prevented the coalescing 

 of the germ ring, and yet produced an increased accumulation of 

 entoderm along the median line. The yolk in this instance appears 

 to lie in a perfectly normal position. In the specimen from which 

 Figures 3 and 3' were drawn, there is a decided difference between 

 the streak of the embryonic region and that of the area opaca, — 

 a difference which was evident on surface view. This difference 

 would seem to argue for the interpretation of Duval and Hertwig. 



2. TJie Concavity of tJie Crcscoitic Opoiing is directed posteriad. 

 — Hitherto we have considered sections through the region of the 

 primitive streak of embryos whose dorsal notochordal crescentic 

 opening has not yet bent posteriad. We proceed now to those 

 stages in which the crescentic opening is bent posteriad, so as to 

 include in its concavity that portion of the streak which has been 

 considered by several reptilian embryologists to be homologous 

 with the amphibian yolk plug. Embryos of this stage fall dis- 

 tinctly into two groups, namely, those embryos along whose 

 streaks a groove extends up to the open blastopore, — a groove 

 which can be recognized on surface view, or in section, or both, — 

 and those whose posterior blastoporic horns have sunk so low 

 into the streak that the central plug-like area has lost its primitive 

 groove. 



a. CJielydra serpentina. — Figures 4 and 4' (Plate I.) are surface 

 views, dorsal and ventral respectively, of an embryo Chelydra ser- 

 pentina. From a ventral surface view it is seen that the noto- 

 chordal canal has opened ventrally. Upon dorsal view the horns 

 of the open blastopore are seen to be bent somewhat posteriad, 

 and to embrace the so called yolk plug region. The posterior 

 bending of the horns is however as yet very slight. In the mid- 

 dorsal line a distinct groove leads posteriad from the blastopore 

 across the so called yolk plug region, and continues backward 

 along the whole surface of the streak. 



Mitsukuri ('93, Plate VII. Fig. 8) figures a similar condition, 

 which he considers to be teratological. In his example, however, 

 the groove does not reach the posterior end of the embryonic shield. 

 Will ('93, Taf. 33, Fig. 9^ p. 561) has figured and described a con- 

 dition of the blastopore of Cistudo lutaria, which he considers to 

 be a condition intermediate between that of a blastopore of the 

 gecko figured by him ('92'', Taf. i, Fig. 7), and Kupffer's Coluber. 

 This Cistudo blastopore possesses a short posterior extension. 



