112 
pricking experiments, IT found it to be confirmed in the way 
I had expected. A prediction from phylogenetic consider- 
ations was thus verified by experiment! 
Gastrulation. — The pricking experiments proved to be 
a very valuable help also in studying the gastrulation in 
Chordates. Extremely divergent opinions have been held 
and are still held regarding this process. Hardly two authors 
agree on the questions as to what is the gastrulation in 
Vertebrates and as to how it is performed. 
We shall first consider the question as to what we have 
to understand by the gastrulation and as to which stage 
is to be designated as the gastrula in Vertebrates. The 
gastrula in Invertebrates is the stage in which two layers 
may be distinguished, the primary ectoderm and the primary 
entoderm, which differ from each other in physiological, 
histological and topographical respect. Thus the gastrula 
is the two-layered stage while the blastula may be designated 
as the one-layered stage. While the latter is represented 
in a permanent state by Volvox (HUXLEY, 1877), the two- 
layered stage owes its phylogenetic significance to the 
comparison with the Coelenterates, first made by HUXLEY 
(1849) who again compared the two primary layers of the 
developing Vertebrate egg with the two layers of the 
Coelenterate, termed ecto- and entoderm by ALLMAN (1855, 
p. 368). The entoderm may originate by delamination or 
by invagination, both being forms of one and the same 
process of which 1 feel inclined to consider the latter as 
giving the purest expression of the phylogenetic process 
of which they are the recapitulation. Here the epithelial 
connection of the cells is preserved during the gastrulation 
process while in the former it gets temporarily lost and is 
only reestablished afterwards. On this question, however, 
we shall not insist here. 
Thus the gastrula-stage is a stage found in the devel- 
opment of all Metazoa — though sometimes modified into à 
form which makes it difficult to be recognized — in which 
part of the epithelium of the blastula has sunk away from 
the surface by a process of invagination or delamination. 
It now clothes as an internal epithelium the archenteron 
which opens to the exterior by the narrow blastoporé. 
Many Vertebrate embryologists are accustomed to advocate 
their views on the gasfrulation and the formation of the layers 
in Vertebrates — often forming their deductions after the in- 
