68 J. B. JOHNSTON, 
within the cartilage this can scarcely be the case, unless the mass 
of packing tissue weighed down the cerebellum after the death of 
the fish and before the hardening fluid had taken effect. I am in- 
clined to regard these differences, which occur in various degrees in 
different brains, as having no especial significance. 
B. The Mid Brain. 
The great height of the cerebellum and tectum as compared with 
other parts of the brain is sharply brought out in Phot. 4. The 
dorsal longitudinal groove is seen in Phot. 5, but is somewhat deeper 
than there appears (compare Phots. 17 and 20), since the membranes 
are not wholly removed. The lateral border of the tectum is marked 
by a slight groove (Phots. 4, 18, 22). Phot. 3 shows the thin tectum 
and the form of the cavity of the mid brain. Running obliquely in 
the lateral wall are seen two ridges. The smaller and more distinct 
one is formed by the right bundle of MEYNERT. The larger, more 
dorsal one is the right limb of the commissura posterior, which is 
seen cut across. This commissure is also seen in Phot. 7, where it 
appears as a U-shaped ridge across the median line. The base of 
the mid brain bends downward as it goes forward, to pass gradually 
into the caudal wall of the lobi inferiores and the thin-walled corpus 
mammillare. 
C. The ’Tween Brain. 
The great size of the hypothalamus as compared with the tha- 
lamus is especially well shown by Phots. 3 and 4. The tectum falls 
abruptly cephalad to reach the level of the epiphysis and commis- 
sura superior which mark the caudal limit of the thalamus. In 
Phots. 1 and 2 the distal part of the pineal stalk is seen extending 
through a narrow canal nearly to the dorsal surface of the carti- 
lage, a considerable distance beyond the cephalic end of the lobi 
olfactorii. The basal part of the stalk can not be seen because it 
runs close over the roof of the ‘‘dorsal sac” (compare Phots. 23—27). 
Phots. 1, 2, 3, and 5 show the form and relations of this sac. In 
Phot. 1 it is intact. In Phot. 5 its roof is removed, showing the 
tela choroidea which forms its floor and separates it from the cavity 
of the fore brain, and showing the opening by which the “dorsal 
sac” communicates with the cavity of the ‘tween brain. In Phots. 2 
and 3 the sac is seen in median section and its relations to the 
fore brain and ‘tween brain are very clear. The roof of the sac is 
