Observations upon the Development of the Teleostean Brain. 489 



a transverse commissure (c. ol.) unites their bases. The olfactory nerves 

 turn outwards. 



Just behind the pineal body a septum (figs. 12, 13 and 14 s.), 

 noticed at the last stage, separates the vesicular tips of the optic ven- 

 tricles (figs. 12 and 13 v. o.). It appears to be formed by an inturuing 

 of their thin roofs, and internally by fibrous matter passing upwards 

 and forwards from the dorsal part of the fibrous tract over the third 

 ventricle {t.f.) to the roof of the optic ventricle. A few large cells 

 appear in its fibrous portion (cf. fig. 12). Ventrally it rests on the 

 fibrous tract (fig. 12 t.f.)\ passing back it becomes thinner, and soon 

 loses its connection with the same, persisting for awhile (figs. 13 and 14) 

 as a cellular leaf-like appendage (s.) of the thin roof of the optic ven- 

 tricle, with a few internal longitudinal fibres. A similar septum, the 

 „central process" of those observers, is described by Mc Intosh and 

 Prince in AnarrMchas (cf. 6, p. 913, pi. XXIII, fig. 5). 



The solid optic nerves are at this stage very thick and longer 

 than before, owing to the outward displacement of the eyes. Fibres 

 may be now very easily traced from their roots into the optic lobes. 



Fibres (fig. 16 fi.) are also seen passing upwards from the cerebral 

 mass across the optic ventricles {v. o.) to the tectum lobi optici {t. I. o.). 



Mc Intosh and Prince noticed (6, p. 908) the flattening of the 

 brain in the Herring of yV inch. This is intensified with age, and is 

 now very strongly marked, the brain being also much elongated. 



A new feature is presented in the appearance of large ganglionic 

 cells in the tori semicirculares (fig. 16 ts.) about the region of the 

 splitting off of the infundibulum (in.). Here the vesicular matter of 

 the cerebral mass is for the most part confined to the dorsal surface, 

 i. e. the floor of the optic ventricle {v. o.) and its central depression, 

 the cerebral fissure. 



White matter forms a very conspicuous dorsal stratum in the 

 tectum lobi optici {t. I. o.), extending to the extreme lateral angle, but 

 falling short of the internal edges of the thick lateral elements of the 

 tectum. Deeply staining cells occur along the dorsal surface of the 

 white stratum, forming a meagre superficial layer, and are sparsely 

 distributed deeper in its substance (cf. 16), in such a manner as 

 to suggest the ditîerentiation of two future cellular layers. Beyond 

 these I can see no signs of the numerous layers found by Stieda 

 (9, p. 40, Taf. 2, fig. 29 a) in Perca. The ganglionic cells of the tori 

 persist back below the valvula (fig. 16 a f.g.) on either side of the 

 aqueduct of Sylvius {aq.). 



