40 Primitive Streak and Notochordal Canal in Chelonia. 



logische Bedeutung, er ist ein Entwicklungshinderniss, er hat 

 keine Funktion. Warum sollte gerade diese Bildung so zah fest- 

 gehalten werden, wahrend doch so vieles andere, das von ungleich 

 grosserer Bedeutung ist, undeutlich wird und verschwindet. Bei 

 den Sauropsiden aber hat sich der Theil des Urmundes, der dem 

 Primitivstriefen der Sauger entspricht, auch schon in eine Primi- 

 tivstreifenbildung umgewandelt. . . . Der Dotterpfropf der Am- 

 phibien ist Darmentoderm, das, wenn es erhalten bleibt, zum 

 Darmentoderm sich umwandelt und sogar zum Entoderm der 

 ventralen Darmwand. Der Zellkomplex, den van Beneden als 

 Dotterpfropf bei Saugern auffasst, jene Zellmasse, welche sich bei 

 verschiedenen Saugern im Grunde der Primativrinne nachweisen 

 lasst, wird aber bei Saugern Mesoderm und liefert auf keinen Fall 

 Material fiir die ventrale Darmwand." 



V. Regression of the Neurenteric Canal along the 



Streak. 



While I believe that the so called plug is only a modified 

 portion of the primitive streak, I cannot entirely agree with Heape, 

 for in the turtle at least this portion of the streak is not in reality 

 "forced up." The plug-like eminence never rises higher than the 

 level of the lateral ectodermal surface. Indeed, it either just equals 

 the lateral ectoderm in elevation, or else falls below the level of it. 

 That such a condition exists is evident not only in many of my 

 own figures, but also in those of Mitsukuri and Ishikawa ('86, Fig. 

 9, Plate III., and Fig. i8, Plate IV.) and Will ('93, Taf. 36, Fig. 24"). 

 Mitsukuri and Ishikawa ('86, p. 28) write as follows : " The con- 

 siderable space between the two lateral lips of the blastopore is 

 filled almost entirely by a plug {yk. p.) of considerable size, which 

 projects upwards from the axial mass of cells as far as the level of 

 the general surface of the embryo." 



This so called plug is formed, not by a forcing upward, but 

 by a sinking_downwardj^ of the streak. Mitsukuri and Ishikawa 

 ('86, p. 80) realized this fact, I believe, when they wrote : " The 

 reason why the yolk plug in Trionyx is more conspicuous at this 

 stage than earlier stands, we think, in close connection with the 

 fact that the blastopore has become a much better defined horse- 

 shoe-shaped slit." This sinking begins in two areas parallel as 

 well as lateral to the axis of the streak, and together with the 



