1 6 Primitive Streak and Notochordal Canal in Chelonia. 



fails altogether underneath the invagination. In other cases for 

 the gecko and the turtle Will is not certain in regard to the 

 method by which the lumen is developed. Yet Will in all his 

 papers, Kupffer (' 82), Mehnert ('92), Mitsukuri ('86 and '93), 

 Wenckebach ('91), Balfour ('79), Weldon ('84), and Strahl ('82) 

 give unanimous testimony to the effect that the lumen proceeds in 

 its development from the posterior towards the anterior end of the 

 shield. 



In regard to the method by which the lumen is developed 

 Strahl ('82, p. 257) writes as follows: — 



" Entweder ist der Canal cine Einstiilpung der obersten Zellenlage nach 

 unten, oder er entsteht durch ein Auseinanderweichen der Zellen des Primitiv- 

 streifen und lage dann also oben im Ectoderm waiter unten im Mesoderm. 

 Dass auch die letztere Entstehungsweise moglich ist, geht daraus hervor, dass 

 ohnedies eine Zellverschiebung bei der weiteren Entwickelung des Canals in 

 Betracht gezogen werden muss, namlich bei dem Durchbruch durch das 

 Entoderm." 



The evidence which the sections of my embryo mentioned 

 above afford leads me to the same conclusion as that to which 

 Strahl came by inference ; namely, that^in j|ome instan ces at leas t, 

 thej^anal is formed by an actual yielding or degeneration of cells 

 in lorn— This may be accomplished by a process of^cornihcation 

 aided perhaps to a limited extent_by resorption. 



IV. Primitive Streak and Dorsal Notochordal Opening. 



I. The Concavity of the Crescentic Opening is directed anteriad. 

 — Up to this time we have concerned ourselves chiefly with the 

 extent of the notochordal canal, that is to say, with its length and 

 width, and have paid little attention to the character of its dorsal 

 opening. The history of the dorsal opening is so intimately 

 bound up with that of the primitive streak, that both will be con- 

 sidered together. 



Mitsukuri ('92, p. 36) has shown that almost as soon as the 

 dorsal opening of the invagination cavity has assumed a transverse 

 position, it becomes a crescentic slit, whose concavity is directed 

 forward and ('93, p. 248) that later this opening bends so that its 

 concavity is directed backward. It is during this last stage that, 

 according to Will ('93), the ectodermic margins of the blastopore 



