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 
(Pl. XXIII. fig. 4), though the folds are less elaborate than those shown by Sriepa 
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 optici, 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 
(Pl. 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. 
