THE TELENCEPHALON IN CYCLOSTOMES 369 
medial olfactory nucleus and the primordium hippocampi. In 
(Chimaera), ganoids and teleosts the evagination has been arrested 
and the eversion of the primordium hippocampi seen in cyclostomes 
is greatly increased. The formatio bulbaris actually bounds the 
median ventricle near the foramen and the apparent folding out of 
of the striatal or somatic area in cyclostomes is absent in ganoids 
and teleosts (see 711 b). 
(d) The primitive relations of the hippocampal primordium 
and of its commissure in the lamina supraneuroporica are splendidly 
clear and instructive in cyclostomes. The selachians present 
essentially the same relations in evaginated brains. In ganoids 
and teleosts (11' b) the extreme eversion has abolished the primi- 
tive supraneuroporic commissure, except in a few forms. In the 
amphibians the commissures are similar to those of ganoids. In 
reptiles and mammals the cyclostome and selachian condition 
reappears. 
(e) The view of the general morphology of the telencephalon 
previously expressed (710 c, 711 a, ’11 b) receives the strongest 
support from these most primitive brains. The functional columns 
of the brain described by the writer (’02 b) extend forward to the 
telencephalon. The ventral columns are represented only by cor- 
relating tissue adjacent to the terminal ridge or optic chiasma. 
The dorsal columns constitute the overwhelming part of the telen- 
cephalon, are greatly hypertrophied and become flexed in conse— 
quence. The dorsal and ventral columns meet in the preoptic 
recess where the sulcus limitans of His ends. The telencephalon 
in petromyzonts ts not elongated parallel with the long axis of the 
fish, but owing to its flexure, is longer dorso-ventrally. 
The olfactory nerve and sac on the one hand and the formatio 
bulbaris on the other are closely related to the neuroporic recess 
and illustrate clearly the principle (711 a, p. 39) that the neuroporic 
recess owes its existence to the attachment of the olfactory nerve 
to the dorsal lip of the neural tube at this point. The evagi- 
nation of hemispheres began here close to the dorsal border at 
some distance from the anterior end of the brain tube. This 
point lies at about the middle or height of the forebrain flexure. 
