of the Fishery Board for Scotland, 



277 



The three primary divisions of the brain — fore-, mid-, and hind-brain 

 — as development advances can be further distinguished, the fore-brain 

 consisting of the cerebrum and the thalamencephalon, the mid-brain 

 forming the optic lobes, and the hind-brain constituting the cerebellum 

 and medulla oblongata. 



The constitution of the brain before the embryo is freed from the egg- 

 membrane has been described by most authors who have written on the 

 embryology of the brain ; and the subsequent changes till before the adult 

 condition is attained will be here considered. 



The Fore-brain. — In the larva of eight days (Plate XL, figs. 1, 2, f.b.) t 

 the cerebrum, for the most part, consists of vesicular matter. In vertical 

 longitudinal sectious towards the side, the cells of the fore-brain are 

 packed closely together. Dorsally, the posterior aspect of the fore-brain 

 is covered by the anterior portion of the mid-brain. At the sides, the 

 first two primary divisions of the brain are indistinctly marked off from 

 each other by the closer aggregation of cells of the first, and the less com- 

 pactly arranged cells of the succeeding mid-brain. Ventrally, the fore- 

 brain is almost divided by a patch of non-cellular substance which does 

 not stain, and which almost corresponds in position with the anterior 

 commissure distinctly seen in older embryos. In median vertical longi- 

 tudinal section, the posterior limit dorsally can be approximately ascer- 

 tained by the position of the pineal body (PI. XL, fig. 2, pn.), and the 

 mass of cells of the pituitary body, pt., appearing on the ventral surface 

 of the brain, marks the hinder portion of the cerebrum. At this stage 

 the anterior cavity is seen in section, and its greatest dimensions is in the 

 dorso-ventral plane. It seems to be filled with a gelatinous-like fluid, 

 which does not respond to stains like Borax Carmine. This cavity is 

 covered dorsally by a layer of cells, two or three in depth, and it is 

 separated from tissue below the brain by a thin layer of cells, in places 

 not above one row in thickness. 



In larvae, two days older (PL XL, figs. 3-11), the cavity in the fore- 

 brain is nearly cruciform, and under low magnification the floor and roof 

 appear so thin (figs. 3, 4) in the most anterior sections of the flexed 

 brain as to be almost imperceptible. This is especially noticeable in fig. 

 3, below the pineal body, and in the region close to the roof of the mouth. 

 The cells of the roof at this point are spindle-shaped, and the tissue be- 

 tween the lateral halves of the cerebrum is also made up of spindle- 

 shaped cells. The cerebral portions are seen in fig. 3, ce., as two lateral 

 cushions, which, by cranial flexure, have become ventral, and they are no 

 longer composed only of vesicular matter. The vesicular substance is 

 made up of round cells interrupted by two distinct areas of fibres, one 

 being found on either side. The fibrous tracts, which are uncoloured in 

 the drawings, separate each of the two lobes of the cerebrum towards the 

 outer sides into a dorsal and a ventral cellular portion. The fibres of 

 these tracts remain distinct through several sections behind fig. 4. They 

 become united to each other across the middle line, leaving a small 

 ventral cap of nerve cells close to the roof of the mouth. The tracts are 

 continued posteriorly and laterally into the optic thalamus, maintaining 

 a position external to the vesicular matter, and form in sections further 

 back (PI. XL, fig. 5) a lateral covering to the cellular substance of the 

 fore-brain and the anterior dorsal part of the mid-brain. 



The optic thalamus comes to lie on the dorsal side of the fore-brain 

 (PI. XL, fig. 4, t.o.). Its position, dorsal to the cerebrum, is brought about 

 by the great extent of the cranial flexure in the larva. Wnile both 

 cerebrum and optic thalamus at this stage are in the form of lateral 

 cushions, the cerebral structures are larger than the latter. As in the 

 lobes of the cerebrum, a fibrous tract is seen in each half of the optic 

 T 



