94 
GEORGE E. NfCHOLLS. 
voof in fore-, mid- nnd hind-brain brain has developed a -? 
nervous layer over its whole extent which may become It 
exceedingly thick. At tlie same time the thickening of the 
walls of the brain has proceeded to such an extent that, as 
already stated, the third ventricle has been almost wholly, 
and the iter largely, obliterated. 
In the region of the optic lobes it appears probable that 
the thickening must have first reduced the optocoel to a 
flattened circular or discoidal space. The apposition of the 
opposite walls over a considerable area in the middle of this 
space must have followed. In the result the optocoel has 
become a moi'e or less annular cavitj’. .Several stages in the 
reduction of the optocoel may be observed in the brains of 
mv specimens of Bdellostoma. 
In the specimen, alreadly referred to, in which the ventri- 
cles have been least reduced, the optocoel is seen to have 
retained this almost perfectly annular shape, d'his same con- 
dition was observed in a second series, which was cut si ightly 
oblicpiely to the sagittal plane, and is lepresented slightly 
diagrammatically in 'I’ext-fig. 7, which was obtained by the 
superposition of camera drawings of several adjacent sections. 
In a third specimen (fig. 21) the posteriorly recurved 
portion of the optocoel no longer opens ventrall}'^ into the 
iter, and a furtlier stage of reduction of this cavity is shown 
in the fourth specimen (fig. 22), where the hinder part of the 
optocoel has been altogethei' suppressed. The latter photo- 
microgi'aph shows particularly well the highly developed 
epithelium (s. c. o.), which lines the sub-commissural canal 
(*■. c. c.) and is continued into the optocoel upon its anterior 
wall, where it clothes the posterior surface of the posterior 
commissure. 
In every case, however, there is a rapid transition from the 
extremely elongated cells of this epithelium into a much 
shorter columnar ependymal epithelium in those parts of the 
lumen remote from the posterior commissure. 
The shoi-t common canal formed by the jnction of the sub- 
commissural and optocoelic canals leads postero-ventrally into 
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