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Part III. — Thirteenth Annual Report 



thalamus, which, in fig. 5, appears to be continuous with the white tract 

 of the cerebrum, the only interruption on either side being a scattered 

 group of cells running outwards and downwards. 



Longitudinal vertical sections of larv^of this age, (Plate XI., figs. 10, 11,) 

 show the distribution of fibrous and vesicular substance at this time. 

 In an obliquely median section, (fig. 11), the anterior and ventral portion of 

 the fore-brain is composed almost entirely of white fibrous-like tracts 

 with embedded cells scattered through the mass. Towards the side, 

 (fig. 10), the white fibrous portion is greatly reduced, the only area being a 

 ventral patch in the region where the anterior commissure (c.a.) is seen 

 crossing from one lobe to the other. A small cap of loose cells is noticed 

 above the olfactory organ, but this appears to be due to the tearing of the 

 tissue in sectioning, and it is not connected with the fibrous tract shewn 

 in fig. 11, as might be imagined. In figs. 10^ the ventral fibrous portion 

 is divided into two^ a short distance in front of the pituitary body, the 

 posterior limb corresponding to the anterior commissure which is more 

 sharply defined in older sand-eels. 



The pituitary body, (Plate XL, fig. 10,^.), which was noticed in larvae 

 two days younger, as a solid mass of cells in close apposition to the 

 tissue at the base of the third ventricle, is now seen as a mass of tissue 

 quite distinct from the basal tissue of the brain. In none of the larvae 

 freed from the embryonic covering does the pituitary body possess the 

 structure which Stieda* describes and figures in Gadus lotus^ (Plate II., 

 fig. 30). In the post-embryonic stages it is a solid accumulation of cells, 

 in close opposition to the base of the brain. In transverse section it lies 

 under the mid-brain. 



In larvae of four days older, (Plate XL, fig. 12, 13f), the cerebrum is 

 arched over by a thin layer of tissue, where the cerebrum (fig. 12ce.), 

 consists of two lateral masses of tissue, partly fibrous and partly vesicular, 

 the cellular portion abutting on the ventricle. The fibrous portion, which 

 is small and rounded in cross section at the most anterior point, increases 

 till it runs the whole length of the dorso-ventral axis of the brain. 

 Behind the section figured 12, it contracts, till it again becomes of small 

 dimensions. In succeeding sections, (fig. \3ce.), the fibrous tract again 

 increases, dorso-ventrally till it is only interrupted as a lateral covering 

 by a group of cells situated at the pointed ventral end. At this level the 

 roof , is thicker, and is entirely vesicular, and the contained cavity which 

 was T. shaped becomes oblong and ovate. 



Larvae four days older, (Plate XIIL, figs. 11, 12,) also show the passage 

 of the cerebrum (ce.), into the optic thalamus (t.o.), and thence into the 

 covering of the hlid-bram (t.l.o.j. The distribution of the fibrous matter 

 in the anterior of the cerebrum (fig. 11, ce.), is the same as that exhibited 

 in Plate XL, figs. 3, 4, ; viz. two round longitudinal tracts in section. 

 The anterior rounded fibrous tract of the optic thalamus is shown on the 

 left of Plate XIIL, fig 11. A similar distribution is exhibited about the 

 /same level in Plate XIIL; fig 12. These tracts appear to be divided into 

 two, one connecting with the white fibrous matter of the tectum lobi 

 optici, and the other continuous with the white fibres of the cerebrum. In 

 three sections taken immediately behind fig. 12, the commissure between 

 both sides of the cerebrum is seen, and it is connected with the veritral 

 part of the tract shown in fig. 12. The cavities of the fore and mid-brain 

 exhibit a shape similar to that in younger forms. Figs. 11, 12 are 



*' Stieda. Studien iiber das centrale Nervensystem der Knockeilfische. ' Zeits^h. 

 f. wiss:;Zool. 1868, Bd. 18, Taf. I. %. II. 



t The sections, figs. 12, 13, have been cut in an oblique direction, the upper part 

 being much further forward than the lower. This accounts for the sections not 

 showing the mid as well as the fore-brain. 



