NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 275 



them Beard, Dohrn, Ayers, and KupflFer, — that the complicated sensory 

 organs of ear, eye, and nose are diflferentiations 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 (SewertzofT, 

 Flirbringer) appears to be variable in dififerent 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 Piatt. I cannot agree with Hoffmann ('9G) and M. Furbringer 

 ('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 PlatVs 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. 



I am 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 boundaiy 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 Piatt ('94) I hold 

 that the otic sense organ was primitively situated above the constriction between 

 van Wijhe's 4th and 5th somites. 



