408 ; A. A. W. HUBRECHT. 
testinal entoderm finds its way into the interior of the 
embryo. This implies that the intestinal epithelium here 
referred to is originally situated elsewhere, and must now 
undergo a transportation by which it is transferred to the 
interior. This petitio principii cannot be allowed to stand, 
and the definition should be formulated so as to cover delami- 
nation as well. It ought, then, to be as follows: 
Gastrulation is a process during which an intes- 
tinal entoderm is differentiated as against an in- 
tegumentary ectoderm, and by which a mono- 
dermic blastos is changed into a didermic. 
This definition is applicable to Invertebrates as well as to 
Vertebrates.} 
We should, however, be very strict in not allowing our 
conception of gastrulation to be further extended to ulterior 
processes that give rise to different organs, and during which 
an undeniable invagination takes place. For that very 
reason those processes have up to now been erroneously 
looked upon as gastrulation. By having persisted in con- 
sidering gastrulation to be necessarily linked to invagination, 
the actual purpose of gastrulation, viz. bringing about the 
didermic stage, has been thrust into the background, and 
undue weight has been attached to the invaginating process. 
We would, however, be quite as little justified to look upon 
the origin of the medullary canal or of the lens and the 
auditory vesicle by local invaginations as a further con- 
tinued gastrulation as we are when we assert that the process 
by which the notochord and the mesoblastic somites are 
brought about is a gastrulation-process. 
During this process true invagination, which has even been 
1 In Haeckel’s ‘ Anthropogenie ’ (4th ed., 1891, p. 156), which may also be 
considered as decisive on this question, we read : ‘ [The cleavage cells] arrange 
themselves in two separate layers, the two primary germinal layers. These 
surround a digestive cavity, the ‘Urdarm’ (primitive enteron), with an 
opening, the ‘ Urmund’ (primitive mouth). The important embryonic stage 
which possesses these oldest primitive organs we call the gastrula, the 
process by which it originates gastrulation.” 
