394 NILS HOLMGREN 
A subdivision of the telencephalon into a hyposphaerium and an 
episphaerium, of which the hyposphaerium receives secondary 
olfactory fibers, the episphaerium tertiary, seems not to be a good 
morphological subdivision. How, then, shall we make a mor- 
phological subdivision of the forebrain, how delimit the pallial 
structures from the subpallial? I think that the morphological 
ground for such a separation is given in the presence of the fovea 
limbica and the sulcus limitans externus, or better the sulcus 
limitans pallii (if present) on the lateral side of the brain-wall and 
the sulcus limitans medialis and the sulcus limitans (Elliot Smith) 
on the medial side and, of course, in the presence of the zonae 
limitantes, those sharply defined, cell-free zones which cut through 
the brain-wall in the neighborhood of the above-mentioned sulci. 
All parts dorsal to the zonae limitantes belong to the pallium, the 
parts ventral thereto are subpallial. This definition coincides 
essentially with that given by Herrick,? but he has not used 
the zona limitans as limit, but taken an imaginary line connecting 
the fovea limbica with the sulcus limitans externus and the sulcus 
limitans medialis with the sulcus limitans (Elliot-Smith). Some- 
times this imaginary line coincides with the zona limitans, some- 
times it falls a little ventral to the zona. This difference seems to 
be of very little practical importance and in cases where the zona 
is lacking, the imaginary line will serve as well as a zona. 
CYCLOSTOMATA 
Petromyzonts 
The forebrain of Petromyzon is very clearly inverted, but the 
medial borders of the inverted parts do not reach each other. 
Thus a pallial septal structure does not occur. Lateral ventri- 
cles are evaginated cephalad in the olfactory bulb, caudad in 
the ‘olfactory lobe.’ In adult Petromyzon ventricular grooves 
are absent, but in the larva there is a distinct sulcus limitans 
externus (fig. 1, s.l.e.) leading cephalad into the bulbar ventricle. 
2 For literature see the list in my paper: Zur Anatomie und Histologie des 
Vorder- und Zwischen-hirns der Knochenfische. Hauptsiichlich nach Untersuch- 
ungen an Osmerus eperlanus. Acta Zoologica, vol. 1, 1920, pp. 137-315. 
