414 A. A. W. HUBRECHT. 
recommend themselves. For that portion of the vertebrate 
embryonic disk which I have proposed to compare to the 
Coelenterate mouth-slit and stomodzum, the name of dorsal 
mouth might be chosen. The difference between the phyla 
of Annelids and Molluscs as compared to Vertebrates is 
thereby all the better marked. 
Provisionally we are not yet enabled to enter into a 
detailed comparison between ontogenetic and phylogenetic 
processes. The possibility of instituting such comparisons 
will probably have disappeared in the graves of the numerous 
transition forms that now rest among the fossils. But the 
chief outlines of the evolutionary process of notochord and 
somites stand out boldly enough and correspond to what 
Sedgwick has first hinted at and what van Beneden has later 
extended but never fully worked out. At the same time not 
only the Diplo- and Hemichordata but also the Cephalo- 
chordata are relegated to a more modest lateral situation 
in the pedigree of Vertebrates. 
The distinction between the head-segment of Polygordius 
and the trunk of the same animal is thus of the same order 
and-belongs to the same category as that between the very 
foremost portion of the body of a Vertebrate and the segments 
that extend behind it. In an earlier publication! I have 
accentuated this distinction by the use of the terms 
“cephalogenesis” and “notogenesis.” I wish to adhere to this, 
and yet to observe that the distinction here intended between 
“kephale” and “‘notos” is not identical with that between head 
and trunk (trunk-segments having been ascertained to enter 
into the composition of the head), but that on one side should 
be ranged the very foremost portion of the head to which the 
ophthalmic and optic nerves belong, whereas on the other we 
place the further subdivisions of the brain with their cephalic 
nerves, as also the basis of the skull with the remains of the 
notochord it contains, the visceral arches and the whole of the 
trunk. 
1 «Furchung und Keimblattbildung bei Tarsius,’ Amsterdam, 1902, K. 
Akademie v. Wetensch. 
