178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



deuce of neuromeres in the primary forebrain also, although the arrange- 

 ment of nuclei does not always conform to the typical condition. 

 Waters ('91, p. 143) says: " In this area [that of the posterior com- 

 missure] the Cod brain shows little or no segmentation, but from the 

 fact that it nearly corresponds in extent to neuromere 11, and that its 

 existence is quite evident' in Arablystoma, it seems probable that this 

 space is occupied by the third and last of the forebrain neuromeres." 

 lu other words, though none of the characteristics of a neuromere are 

 present, it is a priori probable that a neuromere exists here ! 



Orr, Beraneck, and INIiss Piatt have regarded the midbrain vesicle as 

 a single enlarged neuromere. It has an external constriction separating 

 it from its neighbours, a corresponding internal ridge, an inner concavity, 

 an outer convexity, a radial arrangement of cells and nuclei, and in 

 addition is primary in time of appearance. On the other hand, "Waters 

 ('92) says that it is an error to confound the neuromeric segmentation 

 with the so called vesicular segmentation, since he finds in the midbrain 

 region ^^txco^ well marked convolutions of the brain wall," and the 

 characteristic radial arrangement of nuclei, Kupffer ('93*) believes 

 that, since, with Froriep (92") and Zimmermann (91), he finds evidence 

 of three encephalomeres in the midbrain,'^ this confirmation gives a cer- 

 tainty to their results. 



Surely the divergence in the results of other investigators has not 

 proved that Orr, Beraneck, and Miss Piatt were wrong in considering the 

 primary midbrain as a single neuromere, especially since the midbrain 

 and forebrain form parts of a continuous series of primary enlargements 

 of the eucephalou. The majority of investigators (Orr, Beraneck, 

 McClure, Froriep, and Zimmermann) find that the forebrain consists of 

 two neuromeres, without however giving a satisfactory explanation of 

 its marked divergence, in the matter of secondary division, from the 

 typical hindbrain dilatations. If we coimt dorsal expansions, as is done 

 by Waters and others, we may find evidence of at least three neuromeres, 

 which correspond, says Kupffer ('93*), with his Grosshirn, ISTebenhirn, 

 and Schalthirn. Furthermore, if dorsal diverticula be regarded as evi- 

 dence of neuromeres, we must agree with Kupffer that it is impossible 

 to disregard the epiphyses and plexus formations.^ On this basis 



1 Waters says (p. 465) that he thinks McClure is mistaken in assigning to the 

 midbrain region, on purely speculative grounds, a third neuromere. 



- Kupffer found these three secondary subdivisions of the midbrain in Cyclo- 

 stomes, Zimmermann in Selachii, and Froriep in Mammalia. 



8 See Kupffer ('93s p. 519). 



