178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
dence 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 II, and that its 
existence is quite evident in Amblystoma, it seems probable that this 
space is occupied by the third and last of the forebrain neuromeres.” 
In other words, though none of the characteristics of a neuromere are 
present, it is a priori probable that a neuromere exists here ! 
Orr, Béraneck, and Miss Platt 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 “two! 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. A 
Surely the divergence in the results of other investigators has not 
proved that Orr, Béraneck, and Miss Platt 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 encephalon. The majority of investigators (Orr, Béraneck, 
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 count 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, Nebenhirn, 
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. 
2 Kupffer found these three secondary subdivisions of the midbrain in Cyclo- 
stomes, Zimmermann in Selachii, and Froriep in Mammalia. 
3-See Kupffer (’93°, p. 549). 
