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bearing on the problem of cephalic segmentation. The conclusion 
which I have reached is as follows: In 8. acanthias there exists in 
early stages a continuous primitive segmentation serially homologous 
through head and trunk — the “neuromeric” segmentation. In later 
stages there appears in the encephalon a secondary (in time) seg- 
mentation, resulting in the secondary vesicles, which are not serially 
homologous with the segments of the myelon, but give rise to an 
anterior cephalic tract, which is a region sui generis. 
While there is general agreement that the post-otic region of 
the Vertebrate head is composed of segments homologous with those 
of the trunk, there exists great divergence of opinion as to the nature 
and number of the pre-otic segments. RABL und DOHRN represent 
the extremes of opinion concerning the pre-otic region. Where Ras. 
(92) finds not more than three segments, which, however, are of an 
entirely different kind from those of the trunk, DoHrn (90) finds 
twelve to fifteen serially homologous with trunk segments. A more 
conservative opinion, based on both embryological and anatomical evi- 
dence, is that of van WIJHE (82) and GEGENBAUR (88), that the 
pre-otic segments are few and homologous with those of the trunk. 
Although we grant with AHLBORN (84) and Froriep (94) that 
the nervous system is segmented in adaptation to the segmented 
structures connected with it, yet we may also believe that evidence 
of the number of primitive segments in the head may be preserved 
in the nervous system. For, in some Invertebrate embryos segmental 
cephalic ganglia persist even when most traces of mesodermal seg- 
ments and related sense organs have (it is assumed) disappeared. 
It devolves, however, upon one who attempts to elucidate the question 
of cephalic segmentation in Vertebrates by using as criteria the seg- 
ments of the central nervous system, to show the relations of these 
segments to those of the mesoderm and thus their serial homology 
with trunk segments. So far as I am able to see, the only hope of 
determining the primitive segmentation in the encephalon rests on the 
possibility of a comparison of its segments with those of the myelon, 
not only structurally, but also in relation to nervous outgrowths, and 
to those divisions of the mesoderm on which the segmentation of the 
motor nerves ultimately depends. In the following discussion I propose 
(1) to trace the development of neuromeres; (2) to compare the 
structure of the segments of the encephalon with those of the myelon; 
to Dr. E. L. Marx for Petromyzon material and to Miss Juria B. Puarr 
for the use of her preparations of Amphioxus and Necturus. 
