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Part III— Thirteenth Annual Jleport 



IX.— ON THE LARVAL AND POST-LARVAL DEVELOPMENT 

 OF THE BRAIN OF THE LESSER SAND-EEL (Ammodytes 

 iobianus, L.). With Plates XI.-XIII. By J. H. Fullarton, M.A., 

 D.Sc., F.R.S.E. 



In noticing some points in the development of the brain of the Lesser 

 Sand-eel, after that organ shows the typical division of the vertebrate 

 brain into five main portions, I may first allude to the primary conditions 

 which obtain for fishes before the brain can be distinguished into the five 

 portions exhibited in larval, post-larval, and adult life. 



In Osseous fishes the brain at first consists of a median axial plate 

 originating from dorsal epiblast, situated at the anterior end of the body, 

 and in continuation with the spinal chord. In the Sand-eel, as well as in 

 other Teleosteans, this axial plate forms a wedge-shaped mass of cells with 

 the blunted apex pointing downwards and forming the prominent keel- 

 like ridge, which is so characteristic in embryonic osseous fishes. A 

 lumen is formed in this solid axial structure, and the brain is divided by 

 transverse furrows, seen on the upper surface, into the three primary 

 divisions of fore, mid and hind-brain. Till this early stage the primary 

 brain consists of a mass of cells, its walls being several cells in thickness. 

 With advance in growth, fibres begin to appear in the midst of the 

 cellular substance of the brain, and other changes take place which make 

 the brain a more complex organ. 



The general shape of the whole brain-mass, as viewed from the exterior, 

 is a narrow fore-brain ending in a rounded shape anteriorly, a mid-brain 

 joined to the primary fore-brain and widening towards its posterior aspect, 

 where it narrows or becomes contracted to give rise to the primary hind- 

 brain. These three divisions are well marked off from each other by two 

 transverse constrictions, one of which forms a furrow between the fore 

 and mid-brain, and the other between the latter and the hind-brain. 



In the earlier stages the lumen in the brain, from its commencement in 

 the fore-brain, maintains a fairly uniform calibre, but as development 

 progresses, the lumen, though remaining of small calibre at some places, 

 becomes greatly dilated at other parts to form the ventricles or cavities, 

 which remain more or less prominent throughout life. The cavities in 

 the brain serve to mark off the dorsal from the lateral and ventral por- 

 tions. The roof and the floor, which are united at the sides, become 

 more complicated as growth takes place, and advances in development are 

 associated with these changes. 



As might be expected from the embryo being formed along the 

 peripheral surface of a sphere (the egg), its ventral surface is more or less 

 concave, and surrounds the yelk towards the vegetative pole, and its 

 upper surface is convex. But even this flexure observable in the body of 

 the embryo, and partaken in by the embryonic brain, is not sufficient to 

 account for the marked cranial flexure exhibited. This flexure is occa- 

 sioned by the greater growth of the mid-brain, and although it is most 

 marked by the great eminence of the mid-brain, still the flexure, when 

 observed from the side, is greatly reduced during post-larval life. But 

 when vertical longitudinal sections are examined, the floor of the vesicles 

 being taken as axis, shows that the flexure is on the whole increased. 

 The flexure causes the axis to be q -shaped, and from the changes that 

 take place in the floor it ultimately becomes ~ w ~-shaped. By the greater 

 growth of the mid-brain the fore-brain becomes iuferior, so that transverse 

 vertical sections of early stages cut mid- as well as fore-brain. This is 

 rectified afterwards by changes in the floor of the brain (cf. Plate XL, fig. 

 5, XII., fig. 10, XIII., fig. 11, with Plate XIII., figs. 4, 5, 6). 



