GASTRULATION OF THE VERTEBRATES. 407 
this difficulty by emphatically stating (loc. cit., p. 1113) that 
he refers to gastrulation in Vertebrates, etc. He appears 
not to have sufficiently realised that by this limitation to the 
Vertebrates the important generalisation that in the onto- 
genesis of all Metazoa a didermic phase occurs, which we 
term “gastrula,’ is mutilated and loses its value as a gener- 
alisation. 
As to the second definition Keibel rejects it definitely; but 
with respect to the first he feels inclined to be more lenient. 
He holds this definition to be quite acceptable for a zoologist 
who takes into account all the existing Metazoa. This first 
Fic. 2.—A radially symmetrical actinia-like Coelenteratc. 
definition, which was especially patrouised by Lwoff (who 
has given the first energetic impulse towards the clarifying 
of the conception of gastrulation)! suffers, however, by 
the fact that here again Haeckel’s view concerning the 
primary significance of invagination is tacitly admitted.? 
Gastrulation is said to be the process during which the in- 
1 Lwoff, “ Die Bildung der primaren Keimblatter und die Eutstehung der 
Chorda,” ete., ‘ Bull Soc. imp. der naturalistes de Moscou,’ 1894. 
2 I must emphatically assert that the reproach of having included the 
invagination process in the definition of gastrulation does not apply to Keibel 
himself, however much this might seem to be the case if we consider his defi- 
nition No. 1. He has, however, on pp. 1109-1110 of the ‘ Referat’ above 
alluded to, most distinctly stated that he wished this definition to apply to a 
delamination-gastrula as well. 
