of the Fishery Board for Scotland. 



287 



larval forms its cavity stretches forwards (PI. XIII., fig. 8, au.). When the 

 cartilages of the capsule are developed, it presses on the ventral portion 

 of the medulla oblongata, and causes this to become narrow ventrally. 

 At the same time a marked pouching from the exterior is seen constricting 

 the dorsal part of the capsule (PI. XIII., fig. 8, au.). 



In connection with the cerebrum, the olfactory organs are developed. 

 They are seen in early larval stages as a solid ingrowth from the external 

 epiblast of the snout, (PI. XI., figs 3, 4, ol.), and they come to He on the 

 lateral and ventral surface of the cerebrum. In larvae of eighteen days 

 (PI. XIII., fig. 12, ol.), they are seen as an oval mass of cells on the surface 

 and towards their inner end become rounded in section. Their direct 

 connection with the fore-brain at the time is not yet manifest. In older 

 post-larval sand-eels, they appear at the most anterior end of the brain, 

 and they are now open to the exterior. In (PI. XIII., fig. 4, ol.), the 

 cresentic openings are seen, and on the left hand the olfactory nerve 

 passes from the base of the organ to the side of the fore-brain. 



LETTERS 



au. , auditory capsule. 

 au.c, auditory cushion. 

 aq. , aqueduct of Sylvius. 

 c.a., anterior commissure. 

 can., auditory canal. 



cb., cerebellum. 



ce., cerebrum. 

 ci., inferior commissure. 

 c.o., optic commissure. 

 /.&., fore-brain. 

 f.g., fore-gut. 

 h.b., hind-brain. 

 inf., infundibulum. 

 l.i., lobi inferiores. 

 m.b., mid-brain, 

 m.o., medulla oblongata. 



nc. , notoehord. 



ol., olfactory organ. 



OF REFERENCE. 



op. ch. , optic chiasma. 



pa. , pallium or thin roof of forebrain. 



pn., pineal body. 

 pt., pituitary body. 

 r.v.S., recess of third ventricle. 

 sp.c, neural canal. 



t.l., tori longitudinales. 



t.l.o. tectum of optic lobes. 

 t. o, optic thalamus. 



t.s., tori semicirculares. 

 Lv. 4, tectum of fourth ventricle. 



v.c. , valvala cerebelli or fornix. 



v.o., optic ventricle. 



v. 3, third ventricle. 



v. 4, fourth ventricle. 



II. optic nerves, 

 in, oculor-inotor nerves. 



LIST OF DRAWINGS. 

 Plate XI. 



Fig. 1. Vertical longitudinal section through the brain, and auditory capsule of a 



larva of eight days. Z, comp. oc. 8 x Apo. 16 mm. 

 Fig. 2. Similar section in median line, through the brain of the same, as fig. 1. 



Z. comp. oc. No. 8 x apo. 16 mm. 

 Fig. 3. Oblique transverse section through forebrain of a larva of ten days. Z. comp. 



oc. No. 8 x apo. 16 mm. 

 Fig. 4. Similar section through fore and mid-brain, of the same as fig. 3. Z. comp. 



oc. No. 8 x apo. 16 mm. 

 Fig. 5. Similar section through conjoined fore and mid-brain of the same as fig. 3. 



Z. comp. oc. No.8 x apo. 16 mm. 

 Fig. 6. Similar section through optic lobes and lobi inferiores, of the same as fig. 3. 



Z. comp. oc. No. 8 x apo. 16 mm. 

 Fig. 7. Similar section through optic lobe on one side, and medulla oblongata on 



the other, of the same as fig. 3. Z. comp. oc. No. 4 x apo. 8 mm. 

 Fig. 8. Similar section through optic lobe, cerebellum and medulla oblongata of 



the same as fig. 3. Z. comp. oc. No. 4 x apo. 8 mm. 

 Fig. 9. Similar section through medulla oblongata, of the same as fig. 3. Z. comp. 



oc. No. 8 x apo. 16 mm. 

 Fig. 10. Obliquely vertical longitudinal section through fore and mid-brain, 



pituitary body and auditory capsule of a larva of ten days. Z. comp. oc. 



No. 8 x apo. 16 niim 



