DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 913 



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 baud 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 inferior] y, the roof being 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 [i.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 region 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 — causing the cavity before mentioned. The pineal 

 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. 



* What connection this commissural band may afterwards have with the pineal gland is uncertain. 

 VOL. XXXV. PART III. (NO. 19). 7 A 



