912 PROFESSOR W. C. M‘INTOSH AND MR E. E, PRINCE ON 
outer border of the fusiform tori semicirculares, and in part through them, considerably 
encroach on the cavity and bind the roof more closely down. On 20th June a more 
distinct differentiation of the anterior ends of the fore-brain into anterior cerebral and 
olfactory lobes (Pl. XXIV. fig. 4) is noted, and the area behind is more complex, cells 
being developed chiefly on the upper and inner edges, that is, on the sides of the median 
ventricle as seen in transverse sections. The median arch or roof of the fore-brain is 
considerably narrowed in front by the great increase in the nerve-mass at each side, so 
that a mere chink remains. On each side of the central fissure superiorly the margin 
of cylindrical cells trends upwards, and ends in a point, then bending outwards, 
leaves an acuminate projection in transverse section. Externally the arch is com- 
pleted by a layer of tissue containing pigment—continuous with the “pia mater,” and 
internally by another layer, similar in structure, and therefore differing in appearance 
from nerve-tissue proper. This indifferent tissue is probably the anterior boundary of a 
chamber or vesicle which immediately appears in the median line in front of the pineal 
gland, and is possibly homologous with that described by BaLrour and Parker™® in the 
roof of the thalamencephalon in Lepidosteus. The vesicle is preceded by a double papilli- 
form process of the roof, which apparently soon coalesces inferiorly to form the vesicle. 
The entire process is formed of nerve-cells, and when the vesicle is fully developed, these 
present in transverse section a somewhat columnar arrangement, and the wall is sym- 
metrically folded superiorly, The band of nerve-tissue forms the floor of the vesicle or 
chamber just mentioned and the roof of the ventricle, for the anterior lobes present a 
peculiar layer of large nucleated cells under the columnar series lining the thalamencephalic 
chamber. This is especially noticeable in the line of the section shown (under a low power) 
in Pl. XXIII. fig. 3a. The walls of the vesicle are quite distinct from the optic thalami 
at the sides, but gradually (as we proceed backward) they merge laterally into the optic 
thalami, then the roof becomes a mere bridge between them, each end being thoroughly 
incorporated with their tissue; while the floor, assuming a doubly fusiform shape (ve., 
thinner in the middle and at each end), soon disappears. In the drawing (Pl. XXIIL 
fig. 3) the separated ends of this floor are seen—the knife having probably caused 
rupture. When it (the vesicle) first appears in section comparatively little brain tissue 
lies externally and very little inferiorly, while superiorly only the thin roof of the 
vesicle occurs under the pia mater. Proceeding backward the nerve-substance (optic 
thalami) at the sides becomes massive, and the vesicular space enlarges in the median 
line. Moreover, a papilliform process (pineal gland) makes its appearance in the centre 
over the roof of the chamber. A succeeding section (Pl. XXIII. fig. 5, possibly 
somewhat oblique) is instructive, showing the tapering points of the mid-brain touching in 
the middle line, the cellular pineal body appearing immediately beneath, while along the 
roof of the thalamencephalic chamber a commissure connects the central pale fibrous 
region of the commencing optic thalami on each side. An unusual development of 
connective tissue (neuroglia) could only be confounded with this connecting band, and 
* Phil. Trans., 1882, pt. ii. p. 376, 
