270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
following these may seem to favor Gegenbaur’s view that the former be- 
long to a palingenetic portion of the Vertebrate head which ended 
with the 6th (van Wijhe’s) somite (bounding neuromere VII posteriorly 
and ventrally). The structural gap between the seventh and eighth 
neuromeres is not, however, so sharp that it should outweigh evi- 
dences of similarity, and especially the evidence that somites 6 and 7 
are indisputably serially homologous. I must confess that I cannot see 
that the assumption of palingenctic and canogenetic portions of the 
Vertebrate head has added to the clearness of our morphological con- 
ceptions, nor can I see that it is rendered necessary by any ontogenetic 
or phylogenetic evidence now in our possession. ‘Note, furthermore, 
the disagreement of opinion as regards what is and what is not palin- 
genetic or ccenogenetic among those who have been prominent as advo- 
cates of this view, viz. Gegenbaur (’87), his pupil, Fúrbringer (97), and 
Miss Platt (97). While Gegenbaur holds that van Wijhe’s 6th somite 
is palingenetic, Fürbringer regards the 6th, and possibly the 5th and 4th 
somites, as camogenetic. Miss Platt, on the other hand, believes that 
the 4th and 5th somites are palingenetic, but that the 6th somite is 
probably coenogenetic. All this appears to me confusing and unneces- 
sary. The terms conogenetic and palingenetic are purely relative 
terms. I hold the. view that each metamere of the head may be re- 
garded as comogenetic in comparison with the metameres anterior to 
it, the head gradually receiving accessions from the trunk. Gegenbaur’s 
famous “ Kritik” of 1887 appears more an attempt to establish the 
visceral arches as the essential criteria of cephalic metameres, than a 
wholly unprejudiced effort to weigh tho evidence both anatomical and 
embryological which was at his command. I believe that the evi- 
dence given in the present paper tends to strengthen the generally 
accepted opinion, which Gegenbaur has sought to overthrow, that 
the mesomeres in the head, like those in the trunk, afford the most 
trustworthy criteria of metamerism. The dorsal (neuromeric and meso- 
meric) segmentation must be regarded as more conservative than the 
ventral (branchiomeric or splanchnic) segmentation. The lost elements 
are chiefly the ventral ones, Their loss has indirectly caused the losses 
in the dorsal elements, such as the disappearance of splanchnic motor 
fibres from dorsal nerves and (1) of the thickening of the lateral zones of 
encephalomeres I and II. 
It appears to me that the evidence now in our possession gives reason 
to hope for an eventual solution of the head problem, not only as regards 
the nature, but also the number of head segments. The problem, it is 
