4.04, A. A. W. HUBRECHT. 
Lankester can only contribute to eliminate certain difficulties 
which ever and again present themselves when the different 
stages of the ontogeny of vertebrates are analysedand compared. 
Turning back to the very earliest publications in which 
Haeckel and Ray Lankester have formulated their ideas, we 
are immediately struck by a difference in their views which 
has only too often been undervalued, and which concerns the 
earliest phylogenetic origin of the two-layered embryonic stage. 
Haeckel looks upon the process of invagination, Ray Lankester 
upon that of delamination, as the more primitive. I must 
confess that to me Ray Lankester’s view is more sympathetic, 
because I find it easier to imagine that a one-layered, hollow, 
spherical blastula—the starting-point of the Metazoa—has, by 
division of labour, become two-layered in consequence of the 
differentiation in each cell of an ectodermal half (principally 
sensitive and integumentary) and an entodermal portion 
(principally digestive) than that I could be satisfied with the 
proposition that the two halves of the hollow sphere have 
prepared themselves (independently of the natural process of 
division of labour above noted) to bring about an invagination. 
The result of this invagination will, then, also be a defensive 
layer (ectoderm) which encloses a digestive layer (entoderm), 
both together constituting a two-layered stage that has 
originated by invagination, with an opening, the primitive 
mouth (Urmund). 
A two-layered gastrula is thus evolved out of a one-layered 
blastula either by delamination! or by invagination. In the 
second case the primitive mouth is the natural consequence 
of the process of invagination itself: in the first case an 
opening has to become established at a given point in the 
course of time, to which Ray Lankester? has conferred, in 
1875, the name of “ blastopore” (opening, that is, present in 
the blastoderm). 
Asa matter of course the names “blastopore” and “ primitive 
1 In this case the blastocoel gives rise quite naturally to the archenteron. 
2 « On the Invaginate Planula,” etc., ‘Quart. Journ. of Micr. Sci.,’ vol. xv, 
1875, p. 163. 
