THE TELENCEPHALON IN CYCLOSTOMES 343 
The di-telencephalic boundary 
The writer has attempted (’09) an accurate definition of the 
boundary in question, upon the basis of selachian, amphibian, 
avian and mammalian embryos. The result was to show that 
the optic chiasma belongs to the telencephalon, the boundary 
being defined by the velum transversum above and the caudal 
surface of the chiasma ridge below. It was shown that the telen- 
cephalon includes, in addition to the hemispheres, a median por- 
tion surrounding the rostral part of the third ventricle. The 
floor of this telencephalon medium is occupied by the optic 
chiasma. Its roof (roof plate of His) is made up of the lamina 
terminalis, lamina supraneuroporica and tela chorioidea. The 
telencephalon constitutes a complete brain ring or segment as 
His contended, although shorter than His thought. The hemis- 
pheres are lateral evaginations of this telencephalon medium. In 
the series of vertebrates, as in the stages of the ontogeny, a pro- 
gressively larger part of the telencephalon is evaginated into the 
hemispheres, until in man only the chiasma ridge and the small 
region between it and the lamina terminalis remains as the telen- 
cephalon medium. 
This definition of the di-telencephalic boundary has been 
accepted by Herrick (’10), Kappers and Carpenter (711), and others, 
and its substantial correctness will be assumed for the purposes 
of this paper. If the di-telencephalic boundary is to be deter- 
mined in cyclostomes in the same way as in other vertebrates, 
it becomes necessary only to describe the velum transversum in 
cyclostomes accurately and completely, since the chiasma ridge 
is already well understood. 
The recognition by Sterzi of the small fold behind the dorsal 
decussation as the homologue of the velum transversum of higher 
forms was a valuable contribution to the interpretation of the 
cyclostome brain. The velum was recognized, however, only in 
median sections and the postition of the velum in the median 
plane does not define the boundary between the telencephalon and 
diencephalon. The writer has shown at length elsewhere (11 a, 
"11 b) the errors and inconsistencies which arise from taking into 
account only the.position of the velumin the median plane. It 
