756 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



sometimes a little earlier, a fine cleft (mc) appears by separation of the median cells of the 

 encephalon along a vertical longitudinal plane. It commences in the mid-brain, and passes 

 into the fore-brain, extending almost to the anterior limits of the latter (PL IV. fig. 17). 

 This is the first indication of the true neural canal. It passes dorsally, ceasing before 

 reaching the upper surface of the brain, and ventrally, leaving below a thick tract of 

 nervous cells uncleft. The early brain thus becomes incompletely divided, as Ryder 

 aptly expresses it, into "two flat thick plates of cells placed vertically between the eyes" 

 (No. 141, p. 503). At its anterior termination the canal sends off two lateral vertical 

 continuations, forming a cruciform fissure which marks off the fore-brain (fb, PI. IV. 

 fig. 17 ; PI. VI. fig. 6) ; while in the mid-brain, as the fissure ascends, it bifurcates laterally 

 and horizontally, so that the lumen of the mesencephalon, in cross section, is T-shaped 

 (PI. IV. fig. 21), the roof being thinner than the walls and floor, which are very dense, a 

 feature better seen in sections of Anarrhichas (PL XXIV. figs. 3, 6). No continuity of the 

 central canal with the lumina of the optic vesicles seems to be completely established, and 

 certainly no trace of such a connection is observed in sections at this stage. The canal 

 now rapidly extends posteriorly into the trunk, and as it does so vertical lateral cavities 

 are sent off, one pair in front of the cerebral fold, forming the optic ventricle or Iter 

 a tertio ad quartum ventriculum, and a second pair, constituting the fourth ventricle, 

 immediately posterior to the cerebellar fold (cb, PL VI. fig. 6). The most notable 

 feature at this early stage is the continued lateral extension of the mesencephalon (mb), 

 and its progress backward over the metencephalon (cb), until it almost covers the 

 latter with its two broad lobes, which continue to increase in breadth (compare figs. 5 

 and 7, PL VI.). Between the eyes we have, therefore, a prominent mesencephalic 

 dome formed of two halves, narrower in front, but broad and overlapping the narrower 

 posterior fore-brain (thalamencephalon) and the base of the mid-brain. Several days 

 before hatching this extension of the mid-brain takes place, the T-shaped chamber 

 (optic ventricle) increasing in its upper portion and its lateral regions until the roof 

 above exhibits a considerable decrease in thickness and a marked columnar disposition 

 of its cells. 



An embryo before hatching usually shows such a development of the mid-brain as 

 above described (vide PL XIV. figs. 1, 2), and the brain-mass as a whole exhibits that 

 separation and arrangement of its parts which permanently remain in Teleosteans. The 

 mesencephalon embraces the largest extent of the brain, and by its prominence above 

 imparts that rounded bulbous form to the head which is so characteristic of the young 

 fish (PL VI. fig. 7; PL VIII. figs. 6, 7, 10; PL XIII. figs. 1, 5, 6 ; PL XIV. 

 figs. 2, 4). Thus the medulla oblongata (mo), with the anterior transverse fold or 

 cerebellum, forms a hind- brain plate of triangular shape, the mid-brain (mb) constitutes 

 a similar triangular mass, and both have their broader sides or bases towards each other 

 (PL VI. fig. 6), just as Kupffer describes in Clupea (No. 87, p. 220). The cerebellum is 

 almost entirely covered by the posterior enlargements of the optic lobes (op, PL VI. 

 fig. 7), but it protrudes distinctly as a thickened ridge passing across the front end of 



