4.06 A. A. W. HUBRECHT. 
them even the definition of what gastrulation really is cannot 
then be free from prejudice. Such a definition should be 
applicable both to Invertebrates and to Vertebrates, and 
should formulate the origin of the two-layered out of the 
one-layered stage. 
When we look round for the definition which the most 
trustworthy investigators have given of the gastrulation 
of the Vertebrates we immediately observe how heavy a 
penalty has been incurred by not leaving the crucial question 
as to whether delamination or invagination has been the 
origin of the didermic stage of the vertebrate blastocyst 
provisionally unanswered. As a rule gastrulation and in- 
vagination have been looked upon as synonymous. 
All this can be fully appreciated when we open Keibel’s 
very complete and most conscientious ‘ Referat’ on “ Gastru- 
lation und Keimblattbildung der Wirbelthiere” in Vol. X 
of the Ergebnisse der Anatomie und Entwickelungsge- 
schichte (1901). On p. 1111 we read: 
“Three definitions of gastrulation stand out conspicuously 
when among the number that have been given we eliminate 
certain erroneous elements. 
“The first would be: Gastrulation is a process during which 
the cells that will form the intestinal lining find their way into 
the interior of the embryo. 
“The second : Gastrulation is the process during which the 
material for notochord and mesoderm finds its way into the 
interior of the embryo. 
“The third one, given above, would be: Gastrulation is the 
process by which the material for entoderm, mesoderm, and 
notochord finds its way into the interior of the embryo.” 
The citation here given shows clearly that the definitions 
2 and 38 cannot hold good for the Invertebrata, because the 
latter do not possess a notochord. ‘lhe conception of gastru- 
lation is, however, only then of importance when by it we are 
enabled to hold in one grasp developmental phenomena from 
Hydra up to man. 
Keibel, who rejects 1 and 2 and accepts 3, tries to circumvent 
