neal: nervous system in squalus acanthias. 187 



h. Relation of Encephalomeres to Somites. 

 With HofiFmann ('94 and '96) I am able to confirm the presence of 

 van Wijhe's head somites in Squalus (Acauthias) and also Piatt'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 YIIj. A topographic alterna- 

 tion, however, such as that affirmed by Miss Piatt for the hindbrain 

 region of Squalus and Xecturus, 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. 



c. Somatic Value of the Pre-otic 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 ('96), while 

 strengtliening 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 Wijlie. She finds, therefore, it may be inferred, 

 two less hindbrain neuromeres in Necturus than in Squalus. In embryos of Amblj'- 

 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 liindbrain 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. 



^ 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. 



