414 NILS HOLMGREN 
campal, the general pallial, and the pyriform cortices are differ- 
entiated parts of an earlier undifferentiated ancestral cortex, 
which may conveniently be called the primordial cortex. Such 
an undifferentiated cortex is perhaps that of cyclostomes. But 
in that case there is reason for the opinion that the cyclostomes 
are the only representatives of such a pallial type that have sur- 
vived to our days. 
The presence of the same cortical units in the selachians as in 
the higher vertebrates is a fact that throws a new and unexpected 
light upon the phylogenetic development of the forebrain in 
vertebrates, as will be pointed out further on. 
HOLOCEPHALI 
The forebrain of Chimaera 
The forebrain of Chimaera has been regarded (Kappers, 
08; Kappers and Theunissen, 07; Kappers and Carpenter, 
’11) as very important for the understanding of the everted fore- 
brain in teleosts, as being partly inverted, partly everted. To 
this the objection may be made that in teleosts it is the pallium 
that is everted, in Chimaera subpallial parts, the stria medullaris. 
' Thus I think that the forebrain of Chimaera has not the great 
importance for the understanding of the teleost brain as believed. 
The forebrain of Chimaera, however, is very important for the 
understanding of forebrain morphology in lower vertebrates, 
especially in cyclostomes and selachians. There are conditions 
present which help us to understand the phylogenetic develop- 
ment of these brains and also of the brain in Dipnoi. 
The exterior of the Chimaera brain is well known and it is 
enough to refer to the excellent figure given in the ‘Vorlesungen’ 
of Edinger. On this figure, however, the posterior part of the 
telencephalon is lettered ‘Praethalamus,’ a designation which, 
as Johnston has pointed out, is not right. The great length of 
the forebrain is due to the extreme prolongation of the postpallial 
parts of the subpallium which, as Schauinsland has shown in 
Callorhynchus, takes place in the ontogenetic evolution. The 
pallial parts are not subjected to such a prolongation process and 
