GASTRULATION AND EARLIEST DEVELOPMENT 175 



yolk the latter process is replaced more and more by the 

 former, especially in Amniotes. 



Afterwards, however, the supposed invagination, giving 

 rise to the rudiment of the mesoderm and the notochord, 

 was no longer counted to the gastrulation by HUBRECHT 

 (1902, p. 67), who in this respect supports LWOFF and con- 

 sequently advocates the view that in Vertebrates the 

 gastrulation is performed exclusively by delamination (I c. 

 p. 71). An exception must then of course be made for 

 Amphioxus which can no longer be maintained as the pro- 

 totype of Vertebrates. For all Craniates, however, according 

 to HuBRECHT, the rule, cited above, holds: "Sobald der 



Blastoporus auftritt haben wir es nicht mehr mit 



dem Gastrulationsprocess, sondern mit jenem der Bildung 

 des metameren, bilateral-symmetrischen Ruckens und der 

 Chorda zu thun" (HUBRECHT, 1905). 



HUBRECHT (1890, p. 501) even thinks he can disting- 

 uish in Mammalia a posterior ectodermal from an anterior 

 endodermal part of the archenteron-roof, the "proto- 

 chordal wedge" and the "protochordal plate", and afterwards 

 (1908) has tried to extend his conclusions reached in 

 Mammalia to other groups of Vertebrates. In this MARCUS 

 (1910, p. 171) and De LANGE (1912) follow him in their 

 researches on Amphibia where in the same way they disting- 

 uish an anterior endodermal from a posterior ectodermal 

 p?rt of the archenteron-roof. From the former, which 

 according to HUBRECHT corresponds only to the praechordal 

 part of the head (cf. p. 71), DE LANGE (1913, p. 250) derives 

 the primarily unsegmented head mesoderm or "Urmesoderm" 

 of the branchial region which, however, after the conclu- 

 sions reached by us in the foregoing chapter, does not exist 

 but probably represents nothing but the anterior part of the 

 lateral plate. The notochord according to HUBRECHT is derived 

 in its antefior part from the endodermal, in its posterior 

 part from the ectodermal, cells of the archenteron-roof. 



Finally TRIEPEL (1914, 1918, p. 285) considers the archen- 

 teron-roof in Amphioxus to be composed of endodermal, 

 that in Craniates, however, of ectodermal cells. Thus the noto- 

 chord and the mesoderm have a different origin in the two 

 and the Canalis neurentericus proves to be a "Konvergenz- 

 erscheinung". 



Conception arising from my theory. — W {ho. above survey 

 of the conflicting views on the gastrulation and the forma- 



