400 NILS HOLMGREN 
unknown. I, therefore, here will treat of them at some length. 
In the adult Acanthias these cortical structures are not very 
conspicuous, owing to the fact that the cells of the different corti- 
cal parts are not very different in aspect; but in embryos of differ- 
ent ages it is very easy to demonstrate that real cortical layers are 
differentiated. But these structures are already in the postem- 
bryo not very conspicuous, owing to the increase of ganglion cells 
from the ventricular sheet of neuroblastic tissue, and in the 
adult animals they are scarcely more than indicated by more or 
less packed cell formations. To make these structures under- 
stood it is necessary to follow their embryological development by 
means of series of different stages. 
My material consists of the following stages of the common 
Squalus acanthias: 3, 3.3, 3.7, 3.8, 3.9, 4, 4.5, 4.9, 5, 6.5, 8, 15 em. 
length of the body. The brain was dissected out and cut (15 to 
20), especially in the transverse direction. Staining with cresyl- 
violet. 
1. Stage of 3-cm. body length. In this stage in the pallial part 
of the brain the ventricular layer of neuroblastic cells is in great 
activity forming nerve cells which are pressed outward to form a 
thick sheet covering the whole dorsal aspect of the ventricle. 
This layer is not yet delimited from the neuroblastic layer from 
which it is derived. No differentiation is to be found in this 
primordial cortex. 
In the basal parts of the forebrain the cortex of the tuberculum 
olfactorium (area superficialis basalis of Johnston) is about to 
delaminate from the ventricular position in order to form a sepa- 
rate area of cortex-like structure. 
2. Stage of 3.3-cm. body length (measured after fixation in 
Carnoy’s fluid). The primordial cortex is in the stage of delam- 
ination from the ventricular layer. In the foremost part of the 
brain the primordial cortex is continuous with the thick ventricu- 
lar layer, except in its lateral part, where there is a thin cell- 
free space between it and this layer (fig. 5). The delamination of 
the dorsal and medial part of the cortex is not so far advanced, 
but a relatively cell-free space between the cortex and ventric- 
ular layer is marked by a sheet of small cell-free spots which are 
