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 (PI. 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 Balfour 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 PI. 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 (i.e., 

 thinner in the middle and at each end), soon disappears. In the drawing (PI. XXIII. 

 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. 



