CELL MASSES IN THE FOREBRAIN 407 
DIAGONAL BAND OF BROCA AND GYRUS SUBCALLOSUS 
The large basal mass which includes the head of the caudate, 
the parolfactory nuclei, the tuberculum olfactorium and the ante- 
rior perforate space appears on the medial surface as a rounded 
area (figs. 3, 6) bounded dorsally and caudally by a groove which * 
descends to the basal surface, bends transversely, and disappears. 
This groove, as the further description will make clear, is the 
homologue of the fissura prima of His. 
The area dorsal to the horizontal limb of this fissure is the 
primordium hippocampi (Johnston ’13b). The area between 
the fissure and the lamina terminalis (fig. 6, g.s.) is occupied by 
a cell mass and a fiber bundle both of which are characteristic 
of this region in all mammals and perhaps in all vertebrates. 
In a previous paper (713 b) this cell mass has been included in 
the description of the medial nucleus parolfactorius in both the 
turtle and mammals. This was an error. Since this cell mass 
lies caudal to the fissura prima in mammals, it is outside the 
limits of the parolfactory area. It may be spoken of as the 
nucleus of the diagonal band. In mammals this consists of a 
thin plate of closely packed cells which extends from a point 
rostral to the anterior commissure down in front of the optic 
chiasma and caudad on the latero-basal surface to the amygda- 
loid complex. It is very sharply marked in the rabbit and has 
been figured in the opossum and bat (13 b, figs. 28, 42). The 
distinction between this nucleus and the medial parolfactory 
nucleus is shown in figure 46 of the paper referred to, where this 
nucleus consists of small cells rostral to the anterior commissure 
and is not separately labelled. The cell-free zone between this 
and the medial parolfactory nucleus corresponds to the fissura 
prima. 
In the turtle the same plate of cells is found (figs. 20, 21) 
occupying the medial surface rostral to the anterior commissure 
and extending down close in front of the preoptic recess, near the 
large-celled supraoptic nucleus (fig. 19), and continuing later- 
ally and caudally ectal to the medial forebrain bundle (fig. 18) 
until it expands into the large anterior nucleus of the amygdaloid 
