38 Primitive Streak and Notochordal Canal in Chelonia. 



canal of most mammals and lizards open until its definite dis- 

 appearance. Mehnert's statement, that in Emys " verodet das 

 Anfangsstuck des Urdarmcanals ganzlich," is considered by Will 

 as not conclusively demonstrated. 



My collection contains a sufficient number of older stages in 

 which to trace a disappearance and reopening of the canal, if such 

 a condition exists in turtles. 



Figure 6 (Plate II.) illustrates a stage in which the medullary 

 folds are formed, and Figures 12 and 12' (Plate III.) one in which 

 these folds have begun to close dorsally. The lumen of the 

 horseshoe-shaped canal still persists in both cases, but is much 

 smaller than in the stages previously described. This diminished 

 lumen still encloses between its horns a reduced portion of the 

 streak. I believe that at no time does the neurenteric canal of 

 Chelydra serpentina, at least, close to be later reopened. 



Strahl was the first to maintain that the open portion of the 

 blastopore migrates backward along the streak in Lacerta agilis. 

 Sections show, he says, that in spite of the lengthening of the 

 embryo the streak becomes shorter; and since the streak is not 

 absorbed, the neurenteric canal must close in front and open be- 

 hind. This backward progress of the anterior lip of the neurenteric 

 canal resulting from a process of fusion may take place, and yet 

 all traces of such fusion may be so quickly obliterated that any 

 evidence of it is difficult to find in section. 



Agassiz and Whitman ('84, p. 74) have stated that in the case 

 of Teleostei, the whole germ ring is converted into the axis of the 

 embryo. "The concrescence appears under the disguised form of 

 a migratory movement of cells, which accompanies the epibolic 

 growth of the blastoderm." A marginal notch existed in two of 

 their embryos. A notch is almost always distinctly seen on the 

 anterior lip of the notochordal canal of the turtle embryo. Kings- 

 ley and Conn ('83) describe only an inflection of the epidermal 

 layer, while Ryder ('84, p. 565, p. [71] of separate) and Locy 

 ('94, pp. 398-400) have seen metameric segments lying outside 

 the axis in the germ ring of. Elacate and Squalus acanthias re- 

 spectively. Therefore, in the case of those fishes in which concres- 

 cence seems to be a certainty, this process may be so evanescent 

 as entirely or almost entirely to escape observation. 



Of all the theories to account for gastrulation in reptilian devel- 

 opment formulated by embryologists, that set forth by Wenckebach 



