126 
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 (Lc. 
‚ 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 Rückens 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 
art of the archenteron-roof. From the former, which 
according to HUBRECHT corresponds only to the praechordal 
part of the head (cf. p. 258), 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 anterior 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. —Ifthe above survey 
of the conflicting views on the gastrulation and the forma- 
