of the Fishery Board for Scotland. 



279 



taken, the first (fig. 11), through the separate fore-brain and optic cavities, 

 and the latter through the communicating third and optic venticles. 



In post larvae sand-eels, the cavity of the fore-brain is greatly lessened, 

 and approximates the condition found in adults. In specimens, 5 mm., 

 in length (Plate XII., figs. 7, 8), the third ventricle is covered by the thin 

 pallium (pa.), immediately in front of the pineal body (pn.) The 

 pallium at this stage is a thin layer of cells arching over the space 

 between the cerebrum and the pineal body. In longitudinal sections 

 (figs. 7, 8), the cavity of the fore-brain is shown to be a dorsal space, but 

 cross sections, (Plate XII, fig. 10), show that it is a longitudinal space (^.3), 

 dividing the right and left sides of the fore-brain, with lateral diverticula 

 separating the two cerebral lobes (ce.) from the right and left halves of 

 the optio thalamus (t.o.). The shape of the whole cavity, as seen in 

 transverse section is a narrow cross with elongated arms. The anterior 

 commissure is sharply marked off from the vesicular part of the brain, 

 and is narrower in the central region than towards the sides (Plate XII,, fig. 

 7, 8, c.a.). The fibrous and vesicular portions of the fore-brain in post- 

 larval specimens, exhibit relations similar to what is found in larval 

 forms. 



In sand-eels, 12 mm. long, the two cerebral lobes at their anterior end 

 show two fibrous patches on either side of the middle line, (Plate XIII., 

 fig. 4, ce.), and anticipate the early formation of the olfactory lobes. The 

 brain has now undergone a change in the relative position of the parts to 

 one another, and the flexure which was so marked in larval stages has 

 become lessened, so that it is now possible to obtain vertical transverse 

 sections of the fore-brain by itself. With this change in the cranial 

 flexure, the fore-brain apparently becomes compressed laterally, and at 

 places the two lobes of the cerebrum are divided off from one another by 

 a very narrow, but deep canal (Plate XIII., fig. 5, v, 3). This canal is the 

 third ventricle, and divides the brain where the optic nerves (II.) enter 

 the base of the brain nearly into two symmetrical halves. The distribution 

 of the fibrous matter has become more ventral on the one hand, and more 

 dorsal on the other. 



Associated with the fore-brain are certain structures which are pre- 

 formed during embryonic life, and undergo further development during 

 larval and even post-larval life. The pineal body is developed in the 

 angle between the fore and mid-brain dorsally, and the pituitary body 

 arises on the ventral surface above the roof of the mouth, and comes to 

 lie in close contact with the ventral surface of the brain. The pineal 

 body is situated in young larval forms, some distance behind the anterior 

 end of the brain (Plate XL, fig. 2, pn.). At this stage it is a rounded body, 

 composed of a number of cells, and in some forms it appears to fit closely 

 into a small depression on the roof of the brain. It is in such close 

 apposition in some of the earlier larvae (fig. 2, pn.) that the bounding wall 

 between is difficult to make out. But this may be due to the contraction 

 brought about in the manipulation of the larvae necessary to obtain 

 sections. As the flexure of the brain increases, and the fore-brain is 

 arched over by the anterior margin of the mid-brain, the pineal body of 

 the sand-eel, as Holt,* has already noticed in the herring, comes to 

 occupy a more anterior position. Sections through the anterior of the 

 fore-brain (Plate XI., fig. 3) exhibit the pineal body, (pn.), placed dorsally 

 and medianly. When the cranial flexure increases in larvae some days 

 older than fig. 3, the transverse section, cutting both fore and mid- 

 brain, shows the pineal body placed between the thin layers of mid and 



* Holt. Observations on the development of the Teleostean Brain,, with special 

 reference to that of Clupea harengus. Zool. Jahrb. Abth. f. Anat. undOntog. der 

 Thiere, 1891, Bd. iv., Taf. xxix.-xxxi. 



