NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 185 
more. The only evidence which I find of Zimmermann’s anterior “ Hin- 
terhirn Encephalomer” consists of a flexure of the median ventral wall 
appearing in late stages in the anterior portion of the Hinterhirn, 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 figures 
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 neuromeres 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 ; 
lateral, and ventral roots) to them. 
Moreover, the late appearance of the so called neuromeres of the ante- 
not less so, indeed, when we attach hypothetical nerves (dorsal, 
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 afford satisfac- 
tory evidence of a primitive metamerism. Zimmermann (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 offers 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 more 
crowded out by the specializing brain development, and hence appear at 
a much later period in the ontogeny than 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 tho typical hindbrain neuromeres and 
