396 NILS HOLMGREN 
detached cell-groups than of special cortical areas. Thus one 
can state for Petromyzon that there is a cortical layer present ex- 
tending from the ventro-lateral part of the pallium to the medial. 
Such a cortex I would be very inclined to call a palaeocortex, but 
this term is preoccupied by Kappers in another sense. 
At the ventricular wall there is a second layer of cells, forming 
a ventricular cortical sheet (fig. 2, Cort. 2). 
The entire hemisphere receives secondary olfactory fibers 
from the olfactory bulb, but the pallial part receives also tertiary 
olfactory tracts from the subpallial part of the forebrain. There 
is no part present exclusively arranged for the reception of the 
tertiary tracts only. Under the exterior cortical layer there.is a 
stratum containing unmedullated fibers. 
The limit between the pallial part and the subpallial is not 
- marked by a zona limitans, but nevertheless the limit is very 
conspicuous, the cells on the opposite sides of the limit being of 
different dimensions, the dorsal larger, the ventral smaller, but 
more densely crowded. 
In the subpallial part it is impossible to distinguish different 
subdivisions in the arrangement of the cells. The entire basal 
part of the brain acts as a tuberculum olfactorium + stri- 
atum + nucleus taeniae, but no one of these parts is differentiated. 
Is the forebrain of Petromyzon of ancestral type? Almost 
every account of forebrain phylogeny begins with the petromy- 
zonts as having the most primitive structure of all living verte- 
brates. This conception is to some degree justified by the, in 
many respects, ancestral character of these animals and also by 
some features in the forebrain morphology, the forebrain being 
more simply built up than in other vertebrates. There are no 
special subpallial nuclei and the pallium is uniformly constructed. 
But are these features sufficiently significant to allow the con- 
clusion that the entire forebrain is of an ancestral type? I think 
not, and I will now try to show that such a presumption is not 
very well founded. I will try to show that the forebrain of 
Petromyzon is secondarily reduced. 
The ventricular system of Petromyzon consists of a medial 
unpaired ventricle from which the lateral ventricles rise. Each 
