GASTRULATION AND EARLIEST DEVELOPMENT 165 



trulation which, accordingly, in Vertebrates is always 

 performed by delamination (HUBRECHT, 1902, p. 71), though 

 of course an exception must be made for Amphioxus. 

 This, truly, might appear a serious obstacle but it can easily 

 be overcome by no longer recognizing "the holy Amphioxus" 

 (ibid. p. 68) as the most primitive Chordate. 



Thus HUBRECHT (1905), in agreement with ASSHETON and 

 going still one step further than BRACHET, now adheres 

 to the view: "Sobald der sogenannte Blastoporus auftritt, 

 der als Rusconischer After eine Strecke weit um die 

 Eioberflache wandert, um schliesslich vielfach in den 

 definitiven Anus verwandelt zu werden, haben wir es nicht 

 mehr mit dem Gastrulationsprozess, sondern mit jenem der 

 Bildung des metameren, bilateral-symmetrischen Ruckens 

 und der Chorda zu tun". This process in termed "notogenesis" 

 by HUBRECHT. 



Let us now first have a look at the process which in my 

 opinion is the gastrulation of Vertebrates but which according 

 to the above cited authors either indicates the end or comes 

 only after the gastrulation and which is interpreted in entirely 

 different ways. At the surface of the egg this stage is 

 characterized by the formation and the contraction of the 

 blastopore border. Though, as we shall see, hardly two authors 

 agree in regard to the place where this border, e.g. in the 

 Amphibian egg, first appears and how exactly it moves over 

 the surface of the egg, there is now a fairly general agreement 

 that the closure is performed in a rostro-caudad eccentric way 

 and that the dorsal lip which is also the first to appear, in 

 the shape of a crescent, moves much faster than the ventral 

 lip which remains almost stationary. In Invertebrates 

 Rhumbler (1902) reached the conclusion that the entoderm 

 cells particularly play an active role at the gastrulation 

 process and in Vertebrates we get the same impression. 

 Their tendency to sink into the interior of the egg is 

 already evident in the blastula-stage, as is the case in 

 Invertebrates where often the entoderm cells elongate con- 

 siderably before invagination begins. It is also this phe- 

 nomenon which has led BRACHET to his conception of a 

 "clivage gastruleen." Proliferating and growing inwards 

 under the border of the blastopore the entoderm-cells then 

 form the archenteron, and since this process also goes on 

 more actively under the dorsal than under the ventral lip, 

 the part of the archenteron formed here is much more 



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