34 Primitive Streak and NotocJwrdal Canal in Chclonia. 



Figure 55. The depression in the centre of this plug is not pro- 

 longed ventrahvard into a closed fold, as was the groove on the 

 plug of Figure 23 (Plate V.). 



It seems unnecessary to add to this list descriptions of the 

 streak region of other embryos for the purpose of emphasizing the 

 fact that a groove may exist along the axis of the streak up to 

 the-very lip of the notochordal opening, both before and after this 

 opening has begun to bend posteriad. This condition exists too 

 frequently in my collection of twenty-six embryos taken from five 

 genera for me to accept Mitsukuri's explanation ('93, Fig. 8, pp. 

 248, 249) that, in this respect at least, they are teratological. In 

 an earlier paper, Mitsukuri ('86, pp. 29, 30) refers to the fissure in 

 the "Zapfen" of Coluber described by Kupffer. He writes: " We 

 have also observed a similar appearance in some of the earlier 

 embryos of Trionyx, but we are satisfied that there is no true 

 median fissure. What appears to be such is the optical expression 

 of the primitive streak, along which the ectoblast is proliferating, 

 and giving cells to the mesoblast below. Even in the earliest 

 embryos with this appearance, it is doubtful if it ever extends to 

 the extreme tip of the plug. . . . The reason why the yolk plug 

 in Tryonix 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 horseshoe-shaped slit." 



3. Concerning the so called Plug. — A yolk plug, to be in agree- 

 ment with that of the frog, ought to be more clearly defined in 

 early stages. Moreover, Will ('90, p. 599) writes that, after the 

 horns of the blastopore bend backward, " Die Schenkel nehmen mit 

 dem Auswachsen des Primitivstriefs an Lange zu, riicken einander 

 immer naher und naher und bilden so eine Primitivrinne, welche 

 auf der Oberflache des Primitivstriefens verlauft und an ihrem vor- 

 dersten Ende in den Kupfferischen Gang sich hinabsenkt." Again 

 ('92^, pp. 125, 126) he writes: "Der entodermpfropf geht so all- 

 mahlich in den hintern verdickten Theil der untern Urdarmwand 

 iiber, dass es den Bildern Zwang anthun hiesse, wollte man irgend- 

 wie eine Grenze statuiren. . . . Der Entodermpfropf stellt somit 

 auch beim Gecko nur einen Theil der untern Urdarmwand dar, 

 wenn mann nicht vorzieht, sich dahin auszudriicken, dass in Folge 

 des raschen Wachsthums der Urdarmeinstiilpung ein Theil des 

 Entodermpfropfes mit in die Einstulpung hinabgezogen wird." 

 If now we return to Will's ('90, p. 599) earher statement, it seems 



