FOREBRAIN MORPHOLOGY 421 
impossible to make, but using the brain of a 5-em. Acanthias 
embryo, the points of comparison are not difficult to find out. In 
this embryo the cortex consists of two layers, a distal, where the 
cells are tangentially stretched, and a ventricular with perpendicu- 
lar cells. The former corresponds to the cortical layer, the latter 
to the ventricular in Dipnoi. In selachians also the second 
layer delaminates from the ventricular position in the embryo, 
and there is formed a thick cortical plate which wanders up 
against the brain surface. This cellular mass has before been 
named ‘the general pallial cortex.’ To this cortex there is in 
Protopterus and also in Lepidosiren a corresponding formation in 
the dorsally greatly thickened cell-layer, bordering the ventricular 
ependyma dorsal to the ventricle. 
Thus there is an agreement in brain structure in Dipnoi and 
Selachians—an agreement sufficiently great to allow the con- 
clusion that the forebrain types of these groups are to be derived 
from the same ancestral type with inverted forebrain, with at 
least ventricular cortex or perhaps with an outer cortex and an 
inner ventricular cell-sheet. That the last alternative corre- 
sponds better with the facts follows from the statement that 
there are two layers present in Petromyzon. The double cortex, 
thus, is ancestral in the vertebrate series, and the common 
ancestors of Dipnoi and Selachii therefore might have had such 
an one. 
THE EVERTED FOREBRAIN IN POLYPTERUS, GANOIDS, 
AND TELEOSTS 
As I have before pointed out, the everted type of forebrain is 
derived from the inverted, and I have also given some special 
reasons for this opinion. But I think that no other reason is 
necessary than the fact that lowlier fish-types, as cyclostomes, 
selachians, holocephalians, and dipnoans are provided with an 
inverted forebrain. 
As in these primitive fishes the pallium encloses two cell-layers, 
where a subdivision into three parts at least is indicated, it 
follows that these conditions may be traceable in the remaining 
fish-types: crossopterygians, ganoids and teleosts. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 5 
