GASTRULATION OF THE VERTEBRATES. 409 
cinematographed by Kopsch, undeniably takes place, and is 
all the more misleading because in Amphioxus it is in direct 
continuity with the invagination of the gastrula. 
However, when we inquire whether the invagination here 
alluded to contributes towards the origin of the didermic 
phase of development, viz. towards what we have termed 
gastrulation, our answer must be emphatically negative. 
Gastrulation, i.e., the duplication of the germinal layers, is 
terminated before the commencement of the process of in- 
Fie. 3.—A bilaterally symmetrical, elongated, vermiform, actinia-like animal 
with stomodzeum and enteric compartments. 
vagination, by which the bilateral, symmetrical and meta- 
merical vertebrate is completed. This process of invagination 
can be looked upon as a kind of budding (notogenesis), which 
is consecutive upon the formation of the didermic apical 
portion (cephalogenesis). It gives rise to the trunk. 
Moreover the process by which the didermic stage is 
reached in the higher vertebrates, more especially in mammals, 
can be traced with such perfect clearness, that we may 
wonder that in the face of these facts the imvagination 
hypothesis has been so long upheld. The entoderm in 
mammals makes its appearance by a very striking delamina- 
