FOREBRAIN MORPHOLOGY Ae 
ventricle is continued into the lateral wall of the brain as the sulcus 
limitans externus, dorsal to which the pallium is situated. In 
Polyterus the bulbar ventricle also is continued backwards in the 
brain wall by a short sulcus (figs. 37, 38). This sulcus must be 
the sulcus limitans externus. Thus the brain parts dorsal to this 
suleus may contain the pallium. Dorsal to the sulcus there fol- 
lows in the rostral part of the forebrain in Polypterus a short 
ventricular band devoid of nerve cells (fig. 37, 88) covered by a 
simple thin layer of ependymal cells. Lateral to this band the 
brain wall is nearly without cells. In this way a distinct limit 
between the relatively cell-rich subpallial and cell-poor pallial 
parts is formed. This limit has not, except in the medial part, 
the character of a regular zona limitans (z. l. l.), owing to the fact 
that the cells in the pallium are situated at the ventricle, leaving 
the rest of it almost devoid of cells. 
More caudally, where the sulcus limitans externus disappears, 
the cell-free ventricular space is filled up by small cells, and thus 
the ventricular part of the zona limitans also disappears. But 
here begins a deep sulcus, a sulcus limitans pallii lateralis (figs. 37 
38, s.l.p.), marking the ventral limit of the pallium.* It lies 
exactly at the limit between the pallium and the subpallial parts 
and passes along the side of the forebrain almost to its caudal end. 
To some degree this suleus depends upon the direction of the 
basal portion of the pallium, which is more or less inclined against 
the medial line. Asin the caudal end of the forebrain the pallium 
becomes vertical, the suleus becomes less pronounced. 
In the pallium of Polypterus nerve cells are present in two 
different situations. The main part forms a relatively thick 
ventricular layer (figs. 35 to 38), the others are sparsely distributed 
in the dense neuropil mass of the pallium. Thus there are two 
pallial layers as in all hitherto described forms. The ventricular 
layer is subdivided into three parts (figs. 37, 38), a medial (p.c.), a 
dorsal (g.p.c.), and a lateral (h.c.). The medial part of the layer 
° In Amia Johnston has named this sulcus the ‘sulcus limitans hippocampi.’ 
In higher animals the ‘sulcus limitans hippocampi’ belongs to the medial wall 
of the inverted brain. It isnot homologous to the ganoid one. Therefore, I have 
changed its name in Polypterus. 
