Primitive Streak and Dorsal Notochordal Opening. 37 



tive groove on the floor of the notochordal canal by the supposed 

 fact that the streak grows forward, presses into the blastopore, and 

 causes it to assume a horseshoe shape. In the course of rapid 

 invagination a portion of the plug is drawn down into the canal. 

 In agreement with Mitsukuri ('93), I am unable to accept this 

 explanation. Strahl (^"^6, p. 160) produces evidence which leads 

 him to believe that the streak grows backward. 



What is the meaning of this fusion of the halves of the anterior 

 lip of the open blastopore which I have so repeatedly observed in 

 my sections ? What, moreover, is the significance of the median 

 notch which exists in the anterior lip of the blastopore of so many 

 reptilian embryos? Such a notch has been figured by Will for 

 the gecko ; by Strahl, Wenckebach, Weldon, and Balfour for the 

 lizard; by Agassiz and Clark, Kupfifer, Mehnert, Mitsukuri, Hoff- 

 man, and myself for the turtle. 



The primitive groove does not come to lie upon the floor of the 

 notochordal canal by a forward growth and invagination of the 

 streak. Such a forward proliferation would, it seems to me, oblit- 

 erate all traces of the groove. The primitive groove comes to lie 

 temporarily on the floor of this canal, because in the passage 

 posteriad of the neurenteric canal the halves of the anterior lip 

 fuse more rapidly than the ventral lip opens. The rapid differen- 

 tiation of the medullary and notochordal areas would tend quickly 

 to obliterate all trace of fusion of the halves of the dorsal lip, 

 while a groove may persist on the ventral lip for a long time. 



According to Will ('92^, p. 132), there are two neurenteric 

 canals formed in the gecko. The first of these he names Kupfifer's 

 canal. This canal closes and a second neurenteric canal opens 

 later. Concerning the neurenteric canal in Cistudo he writes ('93, 

 p. 577): " Demnach ist es wenn auch nicht vollkommen sicher, so 

 doch hochst wahrscheinlich, dass der Kupfifersche Gang, wie beim 

 Gecko, etwa um die Zeit der Bildung der Medullarinne schwindet, 

 und dass der bei altern Embrvonen aufgefundene weite Canal 

 einen neuen Durchbruch, einen canalis neurentericus s. str. dar- 

 stellt." Van Wijhe ('88, p. 'j6) has found a stage in the develop- 

 ment of selachians in which the neurenteric canal is present, while 

 the blastopore still persists, and according to Goette ('88, p. 161) 

 the prostoma of Petromyzon remains to form the anus. Shipley 

 ('86, p. 331) also testifies to its permanency in Petromyzon. We 

 are accustomed, Will ('92'', p. 132) writes, to find the neurenteric 



