of the Fishery Board for Scotland. 



285 



lie under it. Behind the region of the infundibulum and recess of the 

 third ventricle (PI. XI., tig. 8, r.v.S.), where the notochord is first seen 

 anteriorly (PL XL, fig. 9, n.c), a longitudinal ventral furrow appears op- 

 posite the notochord, dividing the base of the brain into two longitudinal 

 ridge-like prominences. This furrow serves for the reception of the dorsal 

 ridge of the notochord. A plane through the dorsal and ventral longi- 

 tudinal furrows divides the medulla oblongata into two symmetrical 

 halves. Towards the posterior end of medulla, there are numerous nerve 

 cells imbedded in the white fibrous substance, but towards the anterior 

 end the nerve cells similarly placed are few in number. The fibrous 

 tracts which are seen in the base of the medulla are continued forward 

 (PI. XL, fig. 11) to the fore-brain and the vesicular matter has a like dis- 

 tribution on the upper floor. 



In larvae of eighteen days the base of the brain becomes more rounded 

 in the region of the medulla oblongata, and the fibrous tracts are in some 

 places richly supplied with nerve cells (PI. XII., figs. 3 and 4, m.o.). The 

 base of the medulla at this stage, as well as in younger larvae, is much 

 narrower than its upper surface towards the posterior end. This is seen 

 in PI. XL, fig. 9, m.o., and a similar appearance is presented in sections of 

 eighteen days larvae behind (PI. XII., fig. 4). This narrowing is brought 

 about by the pressure exercised by the enlarging auditory capsules (au). 



In post-larval sand-eels of 5 mm., the floor of the brain has been con- 

 siderably thickened from the optic thalamus backwards, and the vesicular 

 area has correspondingly increased (Pl.-XIL, figs. 5-9; PI. XIII., figs. 1-3.) 

 In the fibrous tract which is fairly constant, an increased number of nerve 

 cells is noticed. These nerve cells, which are larger than the cells in the 

 upper part of the floor, are generally present at the point of origin of the 

 different cranial nerves. The chief interruption, however, to the fibres, is 

 in the anterior floor of the brain, where the vesicular matter in the lobi 

 inferiores, and in the neighbourhood of the infundibulum is interposed 

 (PI. XII., figs. 5, 7, 9 ; PI. XIII., figs. 2, 3). 



In older post-larval forms the medulla oblongata is narrow ventrally, 

 especially where the auditory capsule has attained its maximum capacity 

 (PI. XIII., figs. 9, 10). Here the fibrous substance is at a minimum, and is 

 a thin layer bordering the ventral and lateral sides of the medulla. 

 Posteriorly towards the spinal chord (PI. XIII., fig. 10) the fibrous and 

 vesicular substances are very much mixed. Anteriorly, the medulla is 

 broad above, and narrower ventrally, and the fibrous substance is confined 

 more to the ventral prominence (PI. XIII., fig. 8, in.o.). In front the floor 

 of the brain is embraced by the overhanging tectum lobi optici, and the 

 fibres increase and take up in cross-section a large part of the area of the 

 floor (PI. XIII., fig. 7). In the region of the infundibulum the fibrous 

 tract is a central core, being bounded peipherally by cells, those sur- 

 rounding the infundibulum forming the base of the core. 



Below the tectum lobi optici the floor of the mid-brain on either side 

 of the longitudinal furrow, which runs along its middle, shows two ridge- 

 like eminences — the tori semicirculares running parallel to the sulcus. 

 In larvae of ten days (PI. XL, fig. 6, t.s.) they are gently rounded, and are 

 not continued through many sections. The ridges become more pro- 

 nounced as the larvae advance in age, and are developed towards the 

 tectum above. In eighteen days larvae (PI. XII., fig. 1, 2), in the region 

 of the tori longitudinales (t.I.), the ventral eminences (t.s.), almost meet 

 the dorsal ridges, and so between them the lateral parts of the optic 

 ventricle are nearly shut off from the middle portion. Posteriorly the 

 tori semicirculares are not so prominent, as they gradually lessen in height 

 into the floor of the brain. In early post-larval forms of 5 mm., the 

 eminences are also prominent (PI. XIII., fig. 1, t.s.), and a section of a 



