NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 270 
them Beard, Dohrn, Ayers, and Kupffer, — that the complicated sensory 
organs of ear, eye, and nose are differentiations of lateral-line sense 
organs, we may conclude that there exist no fundamental differences in 
nature between pre-otic and post-otic segments. 
The number of cephalic segments in the post-otic region (Sewertzoff, 
Fürbringer) appears to be variable in different Vertebrates. If the 
estimate given by Hoffmann (94) for Squalus be correct, there are six 
post-otic cephalic segments in that form. In the otic and pre-otic 
regions, I hold the number to be not greater than six, and the exact 
numerical correspondence of neuromeres and somites very strongly sup- 
ports the estimate of six, which accords very closely with that made, 
upon similar but not identical grounds, by van Wijhe, Beard, Marshall, 
and Miss Platt. I cannot agree with Hoffmann (96) and M. Fürbringer 
(97), who — from the evidence that there is one more mesodermal seg- 
ment (viz. the “anterior ”) in Squalus and Galeus than in other known 
Selachian embryos — conclude that still other anterior mesodermal 
segments have phylogenetically disappeared, and that it is therefore 
impossible for us to estimate the number of pre-otic segments. We have 
quite as little reason to believe that somites anterior to Platt’s have disap- 
peared, as we have to believe that encephalomeres anterior to encephalomere 
I (the primary forebrain) have once existed. In the exact numerical 
correspondence of neuromeres and somites we have, not only evidence of 
the serial homology of head and trunk segments, but the means to 
determine their number in the pre-otic region. 
IX. Summary. 
Iam unable to regard Locy’s “neural segments ” as segments in the 
true sense of the word, because I find them irregular in size, inconstant 
in number, bilaterally asymmetrical, and without definite relation to 
structures known to be segmental. They are phenomena connected 
with the proliferation and disassociation of the cells of the neural 
crest. 
The posterior boundary of the cephalic plate coincides with the 
posterior boundary of encephalomere VI, opposite which the auditory 
invagination takes place. 
Orr’s criteria for hindbrain neuromeres hold good only for the later 
1 Six neuromeres alternating with five somites. With Miss Platt (’94) I hold 
that the otic sense organ was primitively situated above the constriction between 
van Wijhe’s 4th and 6th somites. 
