62 A, A. W. HUBRECHT. 
Voeltzkow (’99), Schauinsland (’03) in all orders of Reptiles 
seems to me to be the remnant of the space which has always 
been included in the ccelenterate ancestors between the two 
lateral lips of the stomodzeum of the elongating, actinia-like 
ancestral form. ‘That from the wall of this cavity the noto- 
chord develops is only natural; that it communicates—be 
it even in a somewhat whimsical fashion and curiously inter- 
mittently—with the enteric cavity is also nothing but a 
hereditary reminiscence; that laterally it is inclined to tend 
towards communication with protosomites, as Wilson and 
Hill describe for Ornithorhynchus (’07) and Spee (701) for 
Cavia! is again easily understood, if we remember that those 
portions of the primitive ceelenterate enteron which must 
have become the precursors| of a ccelom (separated from an 
enteron) are in immediate continuity with the lower limit of 
the stomodaeym (notochord). 
The fact that this cavity, or slit, or porus offers a different 
extension and different shapes in different vertebrates ; that in 
some it appears as a neurenteric canal, which in others is no 
more visible as an open space; that 1t undergoes a displace- 
ment backwards, and finally disappears, after having appeared 
in different parts of what will be the median plane of the back 
of the animal, shows that during notogenesis this important 
discontinuity of tissue of phylogenetic significance has also 
that particular variability which very old and archaic portions 
of vertebrate organisation so often display (e. g. epiphysis, 
thyroid structures, etc.). 
‘'o look upon it, as van Beneden has attempted, as the 
archenteron, and to degrade the original entoderm to the 
1 The very important question: in how far Wilson and Hill are right in 
stating (’07, p. 117) that the protosomites which they describe and figure 
“have nothing to do with the origination of the first definitive somites, nor 
are they in any way coextensive with the site of differentiation of the latter,”’ 
will not here be entered upon as falling, as the superscription of this chapter 
indicates, outside the scope of this paper. Suffice it to say that my own pre- 
parations furnish me with most useful material for throwing light on these 
very obscure, but, nevertheless, all-important, points in the development of 
mammals, to which I intend to devote full attention on another occasion. 
