FOREBRAIN MORPHOLOGY 411 
23). The cells here form several dense groups connected with 
more scattered cell-formations. 
The pyriform cortex is in its whole extent tolerably well 
separated from the general pallium (figs. 19 to 23) and from the 
lateral olfactory nucleus or the bulbus olfactorius nucleus. Yet 
fusions of all parts are common, but in such cases the cell arrange- 
ment or cell-size makes it always possible to make out the limits 
of each nucleus. Thus it is always possible to find the situation 
of the zona limitans lateralis (figs. 19 to 23). 
2. Subpallial parts. In the septum the pallial parts are 
separated from the subpallial by a rather obsolete zona limitans 
medialis and a sulcus limitans internus. The zona limitans 
medialis passes immediately dorsal to the neuropore. All parts 
below this zona belong to the subpallial septum. 
The subpallial septum is divided into two parts by a very 
conspicuous suleus, accompanied in its hinder part by an ependy- 
mal thickening. This ‘sulcus septalis’ (figs. 21, 22, s.s.) begins 
at the ventral angle of the lateral ventricle. From this it turns 
obliquely upwards and caudal to the front margin of the foramen 
monroi, where it is found immediately below the sulcus limitans 
internus. In the 15-cm. postembryo of Acanthias this sulcus 
septalis is very sharply pronounced. It marks the ventral limit 
of the nucleus lateralis septi (fig. 21, n.l.s.). At the extreme 
caudal portion this nucleus sends up a small tongue above the 
foramen monroi (fig. 22, n.ls.). This represents a_ fimbrial 
structure at least partly homologous with the fimbriain other vertebrates. 
The nucleus medialis septi (figs. 19 to 21, n.m.s.) is found as 
in the preceding stages: in front as a cell-lamina directed upward 
and forming a dorsal continuation of the cortex tuberculiolfactoril. 
In the caudal part this lamina is divided into a dorsal and a 
ventral part, of which the first forms an unpaired cell-mass around 
the base of the processus neuroporicus. The ventral portion, 
however, diverges from the dorsal, and at the level of the front 
part of the foramen monroi it is found in the middle of the basal 
septum, where it is seen in cross-sections as a rather well-delimited 
‘nucleus’ (fig. 22, n.m.s.), lying in the diffusely arranged cell- 
mass which fills up the basal part of the septum. 
