Observations upon the Development of the Teleostean Brain. 481 



and stout olfactory nerves (/, fig. 5) pass to the bases of the first- 

 named (oL). 



Dorsally the cerebrum is cleft by a deep fissure, which partially 

 divides it into two lobes; posteriorly it is shallower, and a narrow 

 tract of fibres, the anterior commissure (c. a.), passes beneath it to con- 

 nect the white matter of the opposite lobes : the roof of the cerebrum 

 is very thin, consisting of a single layer of small cells (pallium of 

 Rabl - Rückhaed) ; it passes behind into the thicker roof of the 

 thalamencephalon. 



The optic thalami are considerable tracts of mingled fibrous and 

 vesicular matter (figs. 1, 2, 3 and 5 t.o.) lying on either side of the 

 third ventricle (v. 3.). They are not sharply defined from the cerebrum 

 in front (ce.) or the mid-brain (m. &.) behind in their basal region 

 (fig. 3), but their dorsal ends project as rounded and wholly vesicular 

 apical prominences (figs. 1 and 2 t. o.) on either side of the pineal 

 body (pn.), clear of either structure. A narrow tract connects the 

 white matter of each thalamus with that of the cerebral lobe in front 

 of it (fig. 3). 



In the very early larval forms the lateral optic ventricles do not 

 appear in the fore part of the mid-brain; the cerebral mass is cleft 

 by a T-shaped fissure (fig. 3), its roof consisting of a thin cellular 

 layer. The horizontal limb persists in later stages, separating the 

 lateral elements of the tectum lobi optici. It becomes constricted be- 

 hind, whilst from the centre of the vertical limb diverticula (fig. 3 v. o.)^ 

 extend outwards into the cerebral mass, representing the first traces 

 of the lateral optic ventricles. 



The white matter of the tectum is seen laterally. 



The angles formed by the lateral optic ventricles with the lower 

 part of the cerebral fissure (fig. 5 t. s.) represent the tori semicirculares. 

 On either side of these there appears a ridge (seen at a later stage 

 in fig. 8 f.g.), which, passing back, extends inwards to unite with its 

 fellow of the other side as the valvula or Fornix of Gottsche, thus 

 roofing over a part of the ventricle as the aqueduct of Sylvius 

 (fig. 6 aq.). 



The optic ventricle extends back laterally to the extremity of the 

 optic lobes (fig. 6 v.o.). The thin hinder walls of the latter are seen 

 passing forwards (fig. 6), into the dorsal limb of the valvula, the ven- 

 tral limb of which passes up as a thick fold, the cerebellum (cb.) 

 tapering rapidly into the thin roof of the fourth ventricle, a con- 

 dition which is shown at a later stage in fig. 10 {f.g. and cb.). 



