NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 187 
b. RELATION or ENCEPHALOMERES TO SOMITES. 
With Hoffmann (94 and ’96) I am able to confirm the presence of 
van Wijhe’s head somites in Squalus (Acanthias) and also Platt’s “ ante- 
rior” somite. Valuable as this repeated confirmation appears to me, 
I regard as equally important the fact that anterior to the sixth (van 
Wijhe’s) somite a mesodermal segment corresponds to each of the primary 
encephalic vesicles (encephalomeres I to VII). A topographic alterna- 
tion, however, such as that affirmed by Miss Platt for the hindbrain 
region of Squalus and Necturus, I do not find. In the early stages of 
development van Wijhe’s sixth somite lies opposite the posterior constric- 
tion of encephalomere VII, but this relation is soon lost. However, the 
numerical correspondence seems important, and I believe that it will be 
shown by a study of nerve relations that the correspondence is not with- 
out morphological significance, 
ce. SOMATIC VALUE or THE Pre-oric MESODERM SEGMENTS. 
Although it has been stated that the purpose of this paper is to dis- 
cuss the nature of the neuromeric segmentation and the relations of 
neuromeres to other segmental structures, it seems to me not incon- 
sistent with this purpose to inquire into the credentials of those meso- 
dermal segments in the Selachian head which van Wijhe in his famous 
paper considered of somatic value. The confirmation of their presence 
in Squalus given by Hoffmann (94 and *96) and myself (796), while 
strengthening the belief in their permanency, which has been greatly 
shaken by the discovery of more numerous segments in other Selachii 
(Torpedo), by no means demonstrates their somatic value.” The dis- 
finds in the hindbrain region two more somites than were seen by van Wijhe (’82) 
and a numerical correspondence of these with the neuromeres, in the latter, on the 
contrary, she finds neuromeres corresponding with a somatic segmentation which 
is the same as that found by van Wijhe. She finds, therefore, it may be inferred, 
two less hindbrain neuromeres in Necturus than in Squalus. In embryos of Ambly- 
stoma I find, in agreement with McClure (’90), no neuromere corresponding with 
encephalomere IV of Squalus, i. e. there is one less neuromere in the Urodele than 
in the Selachian. Now, since I find a numerical correspondence of van Wijhe’s 
somites with hindbrain neuromeres (encephalomeres III-VII) in the Selachian, it 
is clear that they could not likewise correspond in the Urodele. However, I have 
been unable to find evidence of pre-otic somites in Amblystoma, and therefore am 
unable to affirm or deny a correspondence of neuromeric and mesomeric segmenta- 
tion in this form. 
2 It is a matter of great interest that the latest investigation upon Torpedo 
(Sewertzoff, 98) shows that the mesodermic segmentation in Torpedo and Pristiurus 
is the same. Thus van Wijhe’s results receive repeated confirmation. 
