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MESSRS. F. M. BALFOUR AND W. N. PARKER OK THE 
The hind-brain consists of the usual parts, the medulla oblongata and the cerebellum. 
The medulla presents no peculiar features. The sides of the fourth ventricle are 
thickened and everted, and marked with peculiar folds (Plate 25, figs. 47 A and B, m.o.). 
The cerebellum is much larger than in the majority of Ganoids, and resembles in all 
essential features the cerebellum of Teleostei. In side views it has a somewhat 
S-shaped form, from the presence of a peculiar lateral sulcus (Plate 25, fig. 47 A, cb .). 
As shown by Wilder,, its wall actually has in longitudinal section this form of 
curvature, owing to its anterior part projecting forwards into the cavity of the iter A 
This forward projection is not, however, so conspicuous as in most Teleostei. The 
cerebellum contains a large unpaired prolongation of the fourth ventricle. 
II. Development. 
The early development of the brain has already been described; and, although we 
do not propose to give any detailed account of the later stages of its growth, we have 
thought it worth while calling attention to certain developmental features which may 
probably be regarded as to some extent characteristic of the Ganoids. With this view 
we have figured (Plate 24, figs. 44, 45) longitudinal sections of the brain at two stages, 
viz. : of larvae of 15 and 26 millims., and transverse sections (Plate 24, figs. 46 A-G) of 
the brain of a larva at about the latter stage (25 millims.). 
The original embryonic fore-brain is divided in both embryos into a cerebrum (ce.) 
in front and a thalamencephalon (th.) behind. In the younger embryo the cerebrum 
is a single lobe, as it is in the brains of all Vertebrate embryos ; but in the older larva 
it is anteriorly (Plate 24, fig. 46 A) completely divided into two hemispheres. The 
roof of the undivided posterior part of the cerebrum is extremely thin (Plate 24, fig. 
46 B). Near the posterior border of the base of the cerebrum there is a great develop¬ 
ment of nervous fibres, which may probably be regarded as in part equivalent to the 
anterior commissure (Plate 24, figs. 44, 45, a.c.). 
Even in the oldest of the two brains the olfactory lobes are very slightly developed, 
constituting, however, small lateral and ventral prominences of the front end of the 
hemispheres. From each of them there springs a long olfactory nerve, extending for 
the whole length of the rostrum to the olfactory sac. 
The thalamencephalon presents a very curious structure, and is relatively a more 
important part of the brain than in the embryo of any other form which we know of. 
Its roof, instead of being, as usual, compressed antero-posteriorly,+ so as to be almost 
concealed between the cerebral hemispheres and the optic lobes (mid-brain), projects on 
the surface for a length quite equal to that of the cerebral hemispheres (Plate 24, figs. 
44 and 45, th.). In the median line the roof of the thalamencephalon is thin and folded; 
at its posterior border is placed the opening of the small pineal gland. This bod)' is a 
papilliform process of the nervous matter of the roof of this part of the brain, and instead 
* In Wilder’s figure the wails of the cerebellum are represented as much too thin. 
| Vide F. M. Balfour, ‘ Comparative Embryology,’ vol. ii., figs. 243 and 250. 
