Observations upon the Development of the Teleostean Brain. 483 



The dividing commissure, after shutting off the lumen of the in- 

 fundibulum, may be traced for some distance back along the ventral 

 surface of the cerebral mass (crura cerebri); it decreases in size, and 

 is removed from the immediate neighbourhood of the cerebral fissure 

 by the interpolation of a central pillar of vesicular matter. At the 

 point where the commissure is finally lost sight of (fig. 7), the central 

 vesicular matter has assumed the form of a well-defined and restricted 

 dice-box shaped patch of cells (fig. 7 d.h.). These cells, which stain 

 with great intensity, are closely packed. The structure to which they 

 give rise has on either side of it, separated by a narrow fibrous interval, 

 a semilunar vesicular patch {a. s.\ from the upper horn of which a line 

 of cells can be traced (cf. left hand of fig. 7) to the posterior region 

 of the lateral optic ventricle {v. o.), in which region it forms a dorsal 

 boundary of the white matter of the mid-brain. A very slight fissure, 

 which may be detected above this line of cells (fig. 7 fs.), I believe 

 to be due to artificial causes. 



Similar dice-box and semilunar areas are well-marked in the 

 larval Sprat. Though difl'ering slightly in contour and position, they 

 appear to be homologous with the cellular differentiations in the pe- 

 duncular region of the brain of the larval Anarrhichas, described and 

 figured by Mc Intosh and Peince (6, p. 911, pi. XXIII, fig. 6). 



These areas give place posteriorly to a simple median band of 

 vesicular cells, which descends from the cerebral fissure to the ventral 

 edge, and stands out boldly from the white 

 matter, which on either side of it forms 

 the ventral third of the cerebral mass. 

 No white matter is observable in the val- 

 vula and cerebellum at this stage. 



At a point opposite the anterior 

 end of the otocysts (woodcut -fig. 1) the 

 white matter of the ventral portion of 

 the cerebral mass (medulla oblongata) en- Fig. i. Transverse section of 



croaches, on the median vesicular band, ^""^'n «f herring i day old. just be- 



... - •■ 1 hind infuiidibulum. c. commissure, 



and divides it by a very narrow com- / cerebral fissure, g. ganglionic 

 missure (c). <>«"« 



Strands of fibres pass at all points 

 upwards into the vesicular matter, along the ventral edge of which 

 appear large ganglionic cells (g.). Towards the hinder end of the 

 medulla the arrangement of the tissues becomes simpler, and a broad 



