GASTRULATION AND EARLIEST DEVELOPMENT 161 



pricking experiments, I 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 (1853, 

 p. 368). The entoderm may originate by delamination- or 

 by invagination, both being forms of one and the same 

 process of which I 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, whereas 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 a 

 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 blastopore. 

 Many Vertebrate embryologists are accustomed to advocate 

 their views on the gastrulation and the formation of the layers 

 in Vertebrates — often forming their deductions after the in- 



