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 RypER 
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, Pl. LY. 
fig. 17; Pl. 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 
(Pl. 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 [ter 
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. VL). 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. 75 Pl. Vii Sigs. a6.) 7, 105° BL XI, sfiga::d 55,6292 ae 
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 Kuprrer deseribes 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 
