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Part III. — Thirteenth Anmcal Report 



fore-brain. The pineal body as noticed, is a rounded mass of cells, but 

 when post-larval life is reached, it becomes wedge-shaped at its base, 

 (Plate XIII., fig. 5, pn.), and it developes a stalk which runs backwards 

 underneath the tectum lobi optici (Plate XII. fig. 8, pn.). This stalk, 

 which is at first thick, gradually thins out as it is developed in a back- 

 ward direction. At its posterior rounded end in later post-larval life it 

 shows a fibrous structure, the posterior commissure. The pineal body, 

 situated in the posterior roof of the fore-brain, serves to mark off mid from 

 fore-brain. 



The solid proliferation of cells in the ventral region of the fore-brain, 

 that was noticed in Plate XI. fig. 2, pt., is the pituitary body. Like the 

 pineal, the pituitary body is developed during embryonic life, and in larvae 

 it is seen as a solid mass of cells in close apposition to the thin ventral 

 wall of the fore-brain. It lies in the interval between the anterior and 

 posterior limbs of the ventrally prolonged third ventricle (Plate XL fig. 2, 

 pt.) At this stage in longitudinal section its cells are almost indistin- 

 guishable from the cells of the base of the brain, but in larvae two days 

 older (Plate XL fig. 7, pt.), it is distinct from the brain tissue, and lies in 

 open mouth of the infundibulum. It is situated in fact, at the ventral 

 apical end of the prolonged third ventricle (Plate XL fig. 10, v. 3), and it 

 is a solid mass of cells, acting like a plug to the opening of the infundi- 

 bulum. It is rounded in transverse section, but a section through its 

 an tero- posterior diameter shows it to be oval in shape in this direction. 

 During post-larval life when the brain is somewhat flattened, the pituitary 

 body is compressed from above downwards, and in sand-eels of 12 mm., 

 it is markedly oval in cross section, and it is composed of cells closely 

 packed together (Plate XIII., fig. 6, pt.). 



The third ventricle, during early larval stages, is prolonged down- 

 wards to the base of the brain, and its posterior and ventral portion 

 forms the infundibulum. The ventral limb is prolonged posteriorly 

 as the larva advances in age, so that it comes to lie under the base 

 of the mid-brain. The infundibulum of a larva of ten days (Plate XL, 

 fig. 7, inf.), is surrounded by a mass of cells, and its ventral mouth is 

 closed by the plug of tissue which represented the pituitary body. A 

 cul-de-sac canal runs backwards from the sub-ventral third ventricle, and 

 in sections behind the infundibulum it is seen as an oval canal surrounded 

 by vesicular matter, which is separated off from the base of the hind 

 brain. This is the recess of the third ventricle (Plate XL, fig. 8, r.v. 3), 

 the recessus ventriculi tertii of Holt.* The lumen of the infundibulum 

 is prolonged into transverse arms, one of which is seen in the left hand 

 side, in Plate XL fig. 7, inf. In post-larval sand-eels, the lateral arms of 

 the lumen, which during larval life are very short, are greatly lengthened 

 (Plate XII., fig. 9, inf., and Plate XIIL, fig. 6, inf.) The recess, which 

 was oval during larval life, is now in cross section a wide and uniformly 

 shallow canal (Plate XIIL, fig. 7, r.v. 3). The covering of cells around 

 the infundibulum, which was somewhat thick during larval life, has, by the 

 extra pressure exerted on it from below by the cartilages of the base of 

 the brain decreased in thickness. The base of floor of the brain has now 

 become very thin, and a layer of fibres runs between the upper layer of 

 tissue round the infundibulum and the substance of the brain. The 

 recess is continued backwards till the forward end of the notochord is 

 met, and it is inserted above the notochord, i.e. between the latter and 

 the floor of the brain. 



The Mid-brain. — The second primary division of the brain does not 

 undergo such alterations in fish as in the higher vertebrates. In fish 

 while the mid-brain remains comparatively simple, certain changes take 



* Holt. Ut mpra. 



