of the Fishery Board for Scotland. 



281 



place both in external form and in internal structure. In post-larval 

 stages its increasing size and importance is evidenced by the develop- 

 ment of the roof. Morphologically, we may consider it as forming a 

 dorsal and lateral position — the optic lobes ; a ventral part — the floor — 

 which is a continuation of the base of the brain from behind ; and the 

 cavity which places the third and fourth ventricles in communication, 

 and gives off lateral diverticula, forming the optic ventricles. 



The mid-brain in young larvae of eight days, in longitudinal section, has 

 not laterally the compact arrangement of colls exhibited by the fore-brain 

 at the same stage (PI. XI., fig. 1, m.b.). It is slightly distinguished from 

 the anterior division of the brain by a transverse marking, and a trans- 

 verse deep furrow separates it from the hind-brain, especially in the 

 central line. This deep cleft is seen in longitudinal vertical sections 

 (PI. XL, fig. 2), and extends more than half way to the base of the brain. 

 The forward bulging of the mid-brain makes its auterior dorsal portion 

 overlie the postero-dorsal aspect of the fore-brain, so that transverse 

 vertical sections of early larvae through the anterior of the mid-brain 

 (PI. XL, fig. 4, m.b.) also cut the posterior part of the fore-brain 

 dorsally, and the anterior cerebrum ventrally. The anterior and dorsal 

 part of the mid-brain consists of vesicular matter, which lies on either 

 side, and both cellular cushions are arched over by a thin layer of tissue, 

 in which are spindle-shaped cells. These cushions which appear at the 

 anterior end, are the two halves of the tectum lobi optici, which forms 

 such a conspicuous feature in the adult. In sections slightly posterior to 

 this in larvae of the same age (PI. XL, fig. 5, tlo.), the area of the 

 cellular portion is less in proportion to the fibrous part, which forms a 

 dorso-laterai covering to the vesicular matter surrounding the outer border 

 of the large optic ventricle. The fibrous tracts of the tectum are marked 

 off from the lateral and ventral fibrous tracts of the fore-brain by a series 

 of cells running downwards towards the eye. The fibrous area is not 

 continued to the posterior end of the tectum (PI. XL, figs. 6, 7, 8, 10, 

 11, t.Lo.), which is here, as at the anterior extremity (PI. XL, fig. 4, 

 m.b.), made up entirely of cellular matter. The tectum at this time has 

 a pair of longitudinal thickenings on either side of the dorsal central 

 furrows, and they are afterwards very conspicuous in post-larval stages 

 and in adults. These longitudinal ridges are some distance away from 

 the middle dorsal line, and are not to be confounded with the median 

 dorsal ridges — the tori longitudinales — which appear about this stage. 

 In larvae of ten days (PI. XL, fig. 6), the roof of the optic lobes in the 

 middle is merely a thin covering, and towards the posterior end of the 

 lobes this thin roof extends over the greater part of the optic ventricle, 

 v.o. In Plate XL, figs. 6 and 7, the rounded mass of the tectum is shown 

 not to be attached along its whole outer edge to the thin roof, and in the 

 latter figure the backward prolongation of the optic ventricle on the right 

 has been sectioned, and its ventro-lateral tissue is continued into the 

 fibrous area of the base of the brain. The band of tissue connecting it 

 to the basal portion at this level is entirely cellular. In Plate XL, fig. 8, 

 the free margin of the posterior extremity of the optic lobe {t.l.o.) is seen 

 overlapping the cerebellum (cb.). The optic ventricle at this time 

 communicates by wide openings anteriorly with the third ventricle, 

 and posteriorly with the fourth ventricle (PI. XL, fig. 11), the constric- 

 tion between it and the fourth ventricle being at the incipient valvula 

 cerebelli (v.c). 



As the larvae advance in age, besides the lateral thickenings con- 

 stituting the tectum, two other longitudinal thickenings appear below the 

 sulcus on the dorsal surface of the mid-brain (PI. XII., fig. 1, 2, t.l ). 

 These are the tori longitudinales already mentioned. They are formed 



