488 
DR J. STUART THOMSON ON THE MORPHOLOGY OF THE 
indebtedness.* Some of my sections were cut in paraffin, others in celloidin, and 
were variously stained, but chiefly by Mayer’s hsemalum and eosin and by the 
Wiegert-Pal method. The earlier drawings for this paper were drawn with the 
aid of the Edinger Projection Apparatus, for the use of which I have to thank 
Professor G-. Elliot Smith ; the later drawings were made with the Zeiss Projection 
Apparatus, which has been more recently introduced into the Zoological Department 
at Manchester University by Professor S. J. Hickson. The* external characters of 
the brain of Spinax as seen from the dorsal surface are shown in fig. 1, the usual 
parts of the Elasmobranch brain being observed. The brain is of the more slender type, 
and the more massive part of the prosencephalon or fore-brain is connected with the 
olfactory bulbs by slender stalks, the olfactory peduncles. The prosencephalon has 
a fairly deep median groove on the rostral wall and extending over one-half to 
two-thirds of the medio-dorsal and medio-ventral walls. On the dorsal surface of 
the prosencephalon, towards the posterior end, two not very prominent rounded 
swellings or elevations are situated lateral to the part in which there is ho median 
groove ; these are parts of the primordium hippocampi (fig. Id). Anterior to these 
swellings of the primordium hippocampi, a more flattened part of the roof with very 
faint transverse lines is situated ; this area suggests the anterior pallial commissure 
as depicted on the dorsal surface of some Elasmobranch brains, but in the case of 
Spinax this is evidently slightly developed. On the ventral surface of the prosen- 
cephalon, towards the posterior end, a narrow cleft occurs, which is the point through 
which blood-vessels and the nervus terminalis of Locy enter the brain and penetrate 
to the internal tissues. The other points relating to the morphology of the prosen- 
cephalon or fore-brain will be dealt with in Parts I and II of this paper, in which the 
gray masses and the fibre tracts are considered. 
Part I.— THE GRAY MASSES. 
A transverse section through the most anterior part of the free lateral lobe of 
the prosencephalon, and passing through the base of the olfactory peduncle, shows 
a comparatively simple arrangement of the nerve cells or neurones (fig. 2). In the 
lateral lobe of the prosencephalon itself the neurones are distributed in rather an 
irregular manner, without aggregation at any special part. In the peduncular part 
the neurones are aggregated around the immediate margin of the cavity of the 
peduncle, and further at two places of the wall, namely, firstly, near the lateral wall 
opposed to the main mass of the prosencephalon, and secondly, near the dorsal wall. 
These two aggregations of neurones become linked up slightly posteriorly with areas 
of neurones which appear in the prosencephalon lobe proper. These areas in the 
1 I have also to express my indebtedness to the Royal Society of London for a grant of £10 in aid of the 
printing of this paper. 
