THE TELENCEPHALON IN CYCLOSTOMES Se 
ginal or limiting fissure of the hemispheres. It owes its existence 
to the fact that the evagination of the hemisphere causes an angle 
or fold between its wall and the wall of the brain stem. The 
whole fissure may therefore be referred to under the descriptive 
term, stem-hemisphere fissure. When lower vertebrates are 
examined it is seen that the stem-hemisphere fissure is well- 
marked in reptiles and amphibians, but in true fishes is only a 
broad shallow groove or constriction. In Petromyzonts, one of 
the most striking features is the sharp separation between hemis- 
phere and brain stem (fig. 7). In a previous paper (’09) the 
writer has shown that this is not the line of division between telen- 
cephalon and diencephalon. The groove seen in figure 7 running 
from near the median line in front outward and backward is the 
stem-hemisphere fissure. It marks the boundary between the 
hemisphere and the telencephalon medium, not only in cyclostomes 
but in all classes of vertebrates. When the hemispheres expand 
and lie apposed to each other in the median plane, this forms the 
great longitudinal fissure and its lateral extension between the 
posterior pole of the hemisphere and the brain stem. 
Obviously this stem-hemisphere fissure can not be called a 
di-telencephalic fissure. The writer has suggested (’09, p. 516) 
that the di-telencephalic fissure owes its origin to the withdrawal 
of tissue to form the optic vesicle. In this Professor Herrick 
concurs (’10, p. 467). The di-telencephalic fissure les at the 
junction of the telencephalon medium and diencephalon, while the 
hemisphere evagination takes place some distance farther rostrad, 
and the two are entirely independent. That this is so is perfectly 
clear from cyclostomes, selachians and other fishes. The evagina- 
tion of the hemispheres has, therefore, nothing to do with the 
di-telencephalic fissure or the continuity of diencephalon and 
telencephalon. Further, the di-telencephalic fissure is dorsal in 
position from the start and does not extend farther dorsally in 
higher vertebrates. What does happen is that in higher verte- 
brates more and more of the telencephalon medium comes to be 
evaginated into the hemispheres until the stem-hemisphere fissure 
gradually approaches the di-telencephalic fissure. 
