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Part III. — Thirteenth Annual Report 



19 nun. post-larval form (PI. XIII., fig. 7) shows the tori semioirculares as 

 only slight elevations towards the outer surface of the floor. These slight 

 elevations, as they are traced forward, increase in height, and they 

 approach nearer to the middle line. 



The upper part of the base of the mid-brain is composed of cells, and 

 the fibrous portion which is underneath the cellular is only interrupted by 

 the cells bordering on the ventral prolongation of the third ventricle, and 

 by the inferior commissure (PI. XII., figs. 5, 8, c.i.). The backward as 

 well as the ventral development of the third ventricle carries with it the 

 associated parts, so that portions which belong to the more anterior brain 

 come to lie under the posterior. Towards the middle of the base of the 

 mid-brain, the lobi inferiores are developed. In larvse of ten days (PJ. 

 XL, tig. 6, 7, Li.), they are separated off from the base of the mid-brain 

 by a slight development of fibres. In older larvae this line of demarca- 

 tion is more distinct (PI. XII., figs. 3, 4, l.i.), and the inferior lobes are 

 seen in close relation to the infundibulum. In post-larval sand-eels (PI. 

 XIII., fig. 6, l.i.) the lobi inferiores are arched over dorsally and laterally 

 by the tectum lobi optici which, as has been noticed, has become greatly ex- 

 tended. The lobi inferiores consist of a mingled mass of vesicular and 

 fibrous matter. In the region of the extended third ventricle, cells are 

 developed very plentifully in the lobi inferiores, but elsewhere the much 

 fibrous matter is present in their substance. In early larva? (PL XI., fig. 

 6, l.i.) each lobe anteriorly has a patch of white fibrous matter surrounded 

 by vesicular matter, and this again may be bordered by a thin layer of 

 fibres. Towards the posterior (PL XL, fig. 7, l.i. ; PL XII., figs. 3, 4, l.i.), 

 they are composed entirely of vesicular matter, which is indistinguishable 

 from the cellular coat of the infundibulum. At this early stage they are 

 very small, ami during the whole of larval life they are unimportant. 

 In post-larval forms they grow larger, and the same relation of vesicular 

 and fibrous matter met with in younger specimens obtains (PI. XIII., figs. 

 2, 3, l.i.). In later post-larval forms they assume more of the shape 

 which they exhibit in adults, and in sections at some points they are seen 

 as a fibrous mass with scattered nerve cells surrounded by vesicular 

 matter (PL XIII., fig. 6, l.i.). 



Further forward, the base of the mid-brain is continued into the optic 

 thalamus. In early stages, owing to the flexure of the brain, the optic 

 thalamus is seen in section above the cerebrum, (PI. XL, figs. 3,4,5, to.), but 

 its ventral position is made manifest in older sand-eels in connection with 

 the base of the mid and hind brain. I have already described the optic 

 thalamus as it appears in young forms. Into the base of it, the optic 

 nerves are seen to enter, and the decussation of the roots of the nerves is 

 exhibited at PL XII., fig. 2, op. ch. The optic commissure is seen in PL 

 XII., fig. 8, c.o. The crossing optic nerves leave a patch of vesicular matter, 

 below them at the ventral termination of the brain. In post-larval 

 forms, the optic nerves can be traced from their distribution to the eye, 

 to their origin in the base of the brain (PL XIII., figs. 3 and 5, II.), and in 

 the latter figure the nerve fibres of either side can be seen crossing in the 

 base of the brain. 



The auditory organ is developed in connection with the hind brain, 

 and in larval and post larval forms it is seen pressing against the medulla 

 oblongata. In embryonic stages, as in other fish, the first appearance of 

 the auditory organ in the sand-eel, is as a solid mass of cells found near 

 the surface on the dorsal side. But in the stages under review, it is first 

 noticed as a mass of cells, with a small lumen in the centre (PL XL, fig. 

 1, au.). The walls become thinner, and pouches are developed into the 

 central cavity ; these are the auditory canals (PL XL, fig. 10, can.). The 

 capsule lies' in close contact with the lateral wall of the medulla, and in post- 



