NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS, 153 
so far as they have been studied. In both tables it has been impossible 
to exclude much that is theoretical, and in view of this fact general 
conclusions are obviously dangerous. One important result, which 
should be borne in mind during the discussion of the evidence presented 
in this paper, is established, viz. the constancy, in all classes of Verte- 
brates, of five “hindbrain neuromeres ” (“Falten” or “vrais replis”), 
and of their nerve relations. When six have been counted, usually the 
Anlage of the cerebellum has been included with them, and when seven 
(see Hoffmann, *90), another fold behind the true fifth neuromere has 
been counted. There is consensus of opinion that from the third 
“hindbrain neuromere” (designated in Table II. as 5, and as V in my 
figures) the acustico-facialis nerve takes its origin. In counting hind- 
brain neuromeres, then, this may safely be used as a check. In regard 
to the presence of true neuromeres, comparable with those of the hind- 
brain, in the region of the encephalon anterior to the hindbrain, much 
is theoretical, and, as I believe, uncritical. Morphologists have natu- 
rally been more or less prejudiced in favor of the view that a serially 
homologous segmentation extends throughout head and trunk. This 
preconception has led to the search for resemblances at the risk of dis- 
-egarding differences which obviously exist, and as a result structures in 
the encephalon which are morphologically incomparable with the myelo- 
meres have been homologized with them. Moreover, this has been done 
in utter disregard of their relations to the segments of the mesoderm. 
The study of neural segments and their relations to nerves and somites 
in embryos of Squalus acanthias has given me some facts bearing 
on the problem of cephalic segmentation, which are, so far as I know, 
new. The conclusion which I have reached is as follows. Jn S. acan- 
thias there exists in early stages a continuous primitive segmentation of" the 
nervous system serially homologous throughout head and trunk, — the 
“neuromeric” segmentation. In later stages there appears in the en- 
cephalon a secondary (in time) segmentation resulting in the so called 
vesicles, which are not serially homologous with the segments of the 
myelon, but give rise to an anterior cephalic tract, which is a region swe 
generis. 
In the following discussion I propose (1) to trace the development of 
neuromeres; (2) to compare the structure of the segments of the enceph- 
alon with those of the myelon; and (3) to note the relation of the 
neuromeres to the sensor and motor nerves, to the mesodermal somites, 
and to the visceral arches. I shall begin with the deseription of the 
first appearance of neural segmentation in the embryo, 
