DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 3 
there seems no reason to alter the view just mentioned.* Moreover, this commissural 
band of fibres is observed in several succeeding sections as it spreads over the roof of the 
ventricle between the optic thalami, the sides of the chamber being now solid masses 
of nerve-tissue (optic thalami). This section further indicates the opening of the 
thalamencephalic vesicle or chamber into the common (or third) ventricle, extending to 
the anterior lobes in front and the optic thalami in the region under consideration. 
In the next section a cellular band proceeds in the median line of the roof towards the 
margin of the arch, apparently the remnant of the pineal gland formerly mentioned, a 
few of the commissural fibres being still visible beneath. Just behind, the median parts 
of the lobes of the mid-brain become broader, and are separated inferiorly from the central 
process ; moreover, fibres pass between them and through the median process, forming a 
distinct commissure. A space lies on each side of the latter inferiorly, the roof bemg formed 
by the lobes, the floor by a more or less fibrous band on each side connected with the 
median process between the lobes (Pl. XXIII. fig. 5). An indication of a space, possibly 
due to the mode of preparation, occurs below the latter band, viz., at the point marked 
sp. on Pl. XXIII. fig. 5, and then the cells forming the lining of the great central ventricle 
in the thalami occur. As the commissural fibres last spoken of diminish, the central 
body becomes more clearly differentiated from the edges of the lobes of the mid-brain, so 
that it is somewhat awl-shaped, narrower above, where it joins the lobes in the middle 
line, dilating in the free middle region, and again narrowing at its attachment to the 
roof of the inferior region of the mid-brain. In structure this median region is cellular. 
In the next section it is bell-shaped, the handle of the bell being superior. The cells are 
also symmetrically arranged, viz., a thick layer of large cells externally along the margin 
of the bell, and a broad median band of large cells. The latter arrangement, which is 
broadest inferiorly, may indicate a double process (7.e., the coalescence of a structure 
originally double). The sides of the structure become continuous inferiorly with the floor 
of the mid-brain (optic thalami), and its base rests on the commissure (with the probably 
artificial aperture), for separation of the two strands might easily occur in the lax tissue 
(neuroglia). This separation of the commissure into two layers is a marked feature, and 
cells occur between them as they debouch into the thalami (Pl. XXIII. fig. 5). 
Posteriorly the median process becomes more cylindrical, narrows inferiorly, then hangs 
like a leaf-shaped structure from its stalk between the optic lobes, its double nature being 
shown by its pale central recion on each side and the two rows of median cells. It 
diminishes to a mere papilla and then disappears. 
The commissural fibres between the optic thalami (in the larva of 20th June) appear in 
transverse section along with the pineal gland; they are thus in the same plane, and not in 
front of it, as in the Elasmobranchs, and at first they have a median cellular mass, which 

is torn in the preparation studied 
gland lies above the fibres, and below them is the fissure of the mid-brain continuous with 
the Aqueductus Sylvii. The fibres are thickest towards the posterior part of the gland. 
causing the cavity before mentioned. The pineal 
* What connection this commissural band may afterwards have with the pineal gland is uncertain. 
J 
VOL. XXXV. PART III. (NO. 19). CA 
