DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 915 



cells. The folds then increase in complexity, so that in transverse section the 

 contour much resembles the uneven outline of the mucous coat of an alimentary organ 

 (PI. XXIII. fig. 4), though the folds are less elaborate than those shown by Stieda 

 in the turbot. # Meanwhile the two lateral apertures in the lobi inferiores have 

 descended, so that the inner or infero-median wall touches the folded central chamber. 

 Their central area contains cells in a streaked matrix of protoplasm or coagulable sub- 

 stance. The folded mid-region is somewhat triangular in outline, and continues to 

 increase in size, while the lateral apertures gradually diminish, for the cells soon cover 

 the entire area. After the disappearance of the lateral apertures, the infundibulum 

 becomes more elongated transversely, so as to resemble ultimately a transverse bar, and 

 then it disappears. 



About the region of the splitting of the infundibular tissue, and before the crura 

 separate therefrom, the cerebellum appears under the median folds of the optic lobes. 

 It has dorsally the aforesaid lobes, and ventrally the fourth ventricle. 



The organ is marked dorsally and ventrally by a median depression, so that from 

 the first it is bi-lobed, and this condition is soon better marked by the occurrence of a 

 median fissure. It diminishes and disappears posteriorly above the point where the 

 pharyngeal teeth occur in the section. 



When the tip of the cerebellum (valvula) first appears in transverse section below 

 the tecti lobi opticij the massive area beneath has the two cellular differentiations just 

 below the middle, and the trifid fissure of the infundibulum inferiorly. An intimate 

 decussation of fibres takes place in the lower half, transversely as well as vertically 

 (PI. XXIII. fig. 6), in the larval fish of 20th June, while the upper half is split by a 

 median furrow, the edges of which, however, are closely attached. Then signs of 

 separation occur between the upper and the lower divisions, a mass of strong transverse 

 fibres passing below the former, and curving upwards at the sides externally. They close 

 the median furrow superiorly, having beneath them in the same region a mass of grey 

 matter. These commissural fibres increase in bulk as we proceed posteriorly, only a 

 median notch existing in the floor above. This continues backward to the medullary 

 region. 



Shortly after the anterior fold (valvula) of the cerebellum has become distinctly 

 double, a strong band of transverse fibres passes over the roof of the ventricle to the 

 lateral regions. They proceed on each side to the region of the tori, spreading out into 

 the grey matter. After an interval, in which a change in the vault has occurred, for 

 the tecti lobi optici have now become lateral in the sections, while the median line is 

 occupied by the cerebellum and valvula, another strong band of fibres passes across the 

 same region, the direction in the lateral region being chiefly downwards. 



In the cerebellum behind this, various curved fibres cross and vertical bands course 

 from above downward. Towards the termination of the organ a median fissure occurs. 

 Moreover, a bridge of nerve-tissue is thrown over the floor of the ventricle from side to 



* Zeitsch. f. w. Zool, Bd. xviii. p. 44, Taf. ii. f. 30. 



