NEAL: nervous system IX SQUALUS ACANTHIAS. 185 



more. The only evidence which I find of Zimmermauu's anterior " Hiu- 

 terhiru Encephalomer " consists of a flexure of the median ventral wall 

 appearing in late stages in the anterior portion of the Hinterhiru. Since 

 no dorsal or lateral constriction corresponds with this, and since there- 

 fore it cannot be regarded as a vesiculation of the neural tube, I do not 

 consider it as of morphological importance, but explicable simply as a 

 passive result of the flexure of the neural tube. 



Locy ('95, p. 542) finds five "neural segments" in the forebrain and 

 midbrain, — three in the former and two in the latter. He clearly figui'es 

 and mentions in the description of plates, however, the three secondary 

 midbrain expansions described by Zimmermann and myself. 



c. Summary. 



An examination of the literature bearing on the question of neuro- 

 meres in the region anterior to the hindbrain had led me to the conclusion 

 that structures of different morphological value had been described as neu- 

 romeres, and the examination of the secondary subdivisions of the fore- 

 brain and midbrain of embryos of S. acanthias has served to strengthen 

 this opinion. These subdivisions have been shown to differ from the 

 typical neui'omeres in shape, in structure, and in relation to the dorsal 

 and ventral zones of the neural tube. The attempt to establish a serial 

 homology on the basis of such structures alone seems to me quite mis- 

 leading ; not less so, indeed, when we attach hypothetical nerves (dorsal, 

 lateral, and ventral roots) to them. 



Moreover, the late appearance of the so called neuromeres of the ante- 

 rior brain region, together with the fact that they are secondary subdivisions 

 of primary vesicles, and thus differ from the hindbrain and spinal expan- 

 sions, seems a serious objection to the contention that they aff"ord satisfac- 

 tory evidence of a pi'imitive metamerism. Zimmei'mann ('91) attempted 

 no explanation of this difficulty, saying merely that the differentiation of 

 the anterior encephalomeres is retarded for reasons unknown to him. 

 Waters ('92) alone off'ers an explanation. To him it seemed "not unrea- 

 sonable to conjecture that these constrictions, being essentially primitive 

 and in a state of degeneration, have gradually been more and moi'e 

 crowded out by the specializing brain development, and hence appear at 

 a much later period in the ontogeny tlian would be expected." What 

 right, we are tempted to ask, has one to assume the primitive nature of 

 "forebrain neuromeres," in view of the facts that they are late differen- 

 tiations, and that some of them are the fundaments of adult organs, and 

 in this respect differ both from the typical hindbrain neuromeres and 



