THE TELENCEPHALON IN CYCLOSTOMES 347 
aspect of the brain. As indicated inthis figure, that portion of 
the dorsal ventricular space which les above the velum transver- 
sum is properly called the dorsal sac, while the portion below the 
velum belongs to the telencephalic portion of the third ventricle. 
This description of the velum transversum makes it possible 
to define clearly the boundary between the telencephalon and dien- 
cephalon. It is marked, as shown in figures 5 and 6, by a line 
running from the attachment of the velum transversum upon the 
dorso-rostral border of the eminentia thalami to the caudal surface 
of the chiasma ridge. 7 
The ventricular sulci in diencephalon and telencephalon 
Attention has been called above to the sulcus limitans hippo- 
campi, the dorsal or sub-habenular sulcus and the suleus 6} or 
sulcus medius. In figure 5 are to be seen three other important 
ventricular grooves, the sulcus limitans of His, the sulcus hypo- 
_thalamicus and a sulcus connecting the recessus praeopticus with 
the foramen interventriculare. The presence of the last named in 
embryos and adults of other classes of vertebrates has been pointed 
out (ll a). The further study of slightly evaginated brains 
(cyclostomes, ganoids and teleosts) makes it necessary to with- 
draw the view stated earlier (in 711 a, p. 45) that the sulcus arising 
in the preoptic recess is the continuation of the sulcus limitans 
hippocampi. 
The sulcus hypothalamicus is seen a short distance behind the 
interventricular foramen diverging ventrally from the sulcus limi- 
tans hippocampi. It grows deeper and descends into the hypo- 
thalamus as a crescentic groove. Discussion of the question 
whether this sulcus or any part of it is comparable to the sulcus 
hypothalamicus of the human brain is reserved. The name is 
used here ina purely descriptive sense and is evidently appropriate. 
The sulcus is the same as Herrick’s sulcus diencephalicus ventralis. 
This name has not been adopted because the sulcus in all lower 
vertebrates is a transverse rather than a longitudinal sulcus. 
The sulcus limitans of His traverses the midbrain in the same 
position as in other vertebrates and meets the sulcus hypothalami- 
cus over and in front of the tuberculum posterius. It can not be 
