THE TELENCEPHALON IN CYCLOSTOMES 308 
foramen, the other from the ventral angle, and the two converge 
into the neuroporic recess. The lower one of these grooves is the 
one designated by Herrick as the telencephalic extension of the 
sulcus dorsalis (Herrick ’10, figs. 75, 76). The dorso-median ridge 
(my promordium hippocampi) ends rostrally in the sections which 
contain the so-called dorsal olfactory commissure. As seen in the 
model, it is very abruptly reduced in dorso-ventral thickness at 
the foramen and extends over the foramen only as a slender strand 
of cells (which appears in Herrick’s fig. 77) in close connection 
with the commissure. Rostral to the foramen this slender ridge 
disappears entirely and the larger ridge which Herrick calls the 
dorso-median ridge in his figures 75 and 76 contains glomeruli and 
olfactory fibers and belongs to the formatio bulbaris. Of the 
two sulci extending rostrally from the foramen, the upper one 
separates the dorso-median ridge (primordium hippocampi) from 
the formatio bulbaris, the lower one separates the formatio bulbaris 
from the medial olfactory nucleus. — 
In the ammocoetes of Petromyzon dorsatus (figs. 3, 8) there is 
only one groove extending from the foramen to the neuroporic 
recess. This sulcus separates the primordium hippocampi from 
the medial olfactory nucleus and there is no formatio bulbaris 
in this position . Lampetra presents an intermediate condition. 
The two grooves are nearer together and the area of formatio 
bulbaris which abuts on the ventricle isless than in Ichythyomyzon. 
These facts show that in Ichthyomyzon and Lampetra the evagi- 
nation of the hemisphere has not completely carried out the formatio 
bulbaris, but that-a part of this formation remains in the telen- 
cepbalon medium and forms part of the wall of the median ventri- 
cle rostral to the foramen. In Petromyzon dorsatus the evagina- 
tion of the formatio bulbaris is complete. Consequently, the 
two sulci in Ichthyomyzon and Lampetra may be regarded as 
merged into one sulcus in Petromyzon dorsatus. This one sul- 
cus begins in the neuroporic recess and passes into the rostral 
wall of the lateral ventricle, separating the primordium hippo- 
campi from the medial olfactory nucleus. The position of this 
sulcus is the same as that of the medial zona limitans (hippocampi) 
in selachians (Johnston, ’11 a). 
