THE GASTRULATION. QUESTION. 421 
The Gastrulation Question. 
By 
Franz Keibel. 
In consequence of the frequent discussions that took place 
in the course of the summer between Professor Hubrecht and 
myself concerning the gastrulation of the Vertebrates, he 
kindly sent me his paper on this subject for inspection before 
its publication. I would like to adda short explanation to 
his argumentation. 
(1) Hubrecht is undoubtedly justified in demanding that 
the definition of gastrulation should be so formulated that it 
can apply to all the Metazoa. I have already acknowledged 
that this was admissible in my paper: “Ueber die Gastrulation 
und die Keimblattbildung der Wirbelthiere,” p. 1112, with 
special reference to Lwotf,! but then, starting from Amphi- 
oxus, | gave a definition which could only apply to Verte- 
brates (p. 1113). I now hold that this compromise with the 
current views is no longer justified, and on this point I entirely 
agree with Hubrecht. 
(2) I am, further, in perfect agreement with Hubrecht that 
in the definition of the phenomenon of gastrulation the pro- 
1 We read there: ‘‘Dagegen ist die Detinition, dass die Gastrulation ein 
Vorgang ist, bei dem die dem Darm oder genauer das Darmepithel bildenden 
Zellen, in das Innere des Kies gelangen nicht so ohne Wieteres abzulehnen. 
Lwoff der diese Definition vertritt, ist durchaus consequent und da wir bei 
den Wirbeliosen Organismen kenunen welche dem Mesoderm und der Cuorda 
der Wirbelthiere ohne weiteres zu vergleichende Bildungen nicht besitzen, 
ist seine Definition vom Standpunkt der die Gesammtheit der Metazoa 
betrachtenden Zoologen durchaus berechtigt.” 
