494 ERNEST W. L. HOLT, 



it may do so in earlier stages. In a post-larval form (fig. 22) it extends 

 slightly forwards. This is perhaps due to the greater upward rotation 

 of the cerebrum, and the absence of that flattening so noticeable in 

 the Herring's brain. The superior commissure of the blenny (figs. 21 

 and 22 c. s.) is remarkable, in being distinctly double from the earliest 

 stage at which I have studied it. At first the two fasciculi are equal 

 in calibre, but with age the dorsal outgrows the ventral one (cf. figs. 21 

 and 22). In the post-larval stage the apex of the velum (fig. 22 f.) 

 is thickened, and lodges a fine transverse commissural band (co.). 



In the Herring of | inch, a continuous fibrous tract (fig. 12 t.f.) 

 runs back in the middle line between the pineal stalk and the posterior 

 commissure, forming a fibrous bridge over the third ventricle; it is 

 thickened immediately behind the pineal stalk (fig. 12), where it supports 

 the median septum (s.) of the optic ventricle; further back (fig. 13) 

 it thins out, thickening again as the transverse posterior commissure 

 (fig. 14 c.jp.). The tract is seen to be formed by fibres passing up- 

 wards from the optic thalami (figs. 12 and 13 t.o.)^ and then forwards 

 along the middle line towards the pineal stalk. A layer of cells under- 

 lies the tract, and cells at intervals intervene throughout its substance. 

 It has a double nature throughout, which is more easily made out at 

 a later stage, viz ly^^ inch (fig. 19 t. f.). 



This tract, which I propose to term the Labium invaginatum of 

 the optic ventricle, is in reality a backwardly directed fold of the 

 brain-roof, continuous anteriorly with the hind wall of the pineal stalk, 

 and posteriorly with the tectum lobi optici, and thus bears to those 

 structures the same relationships as the valvula does to the latter and 

 to the cerebellum (cf. fig. 18 t. f. and f. g.). The posterior commissure 

 (fig. 19 c.p.) marks the apex of the fold. The great backward ex- 

 tension, thinness, and close adherence of the limbs of the fold must 

 be regarded as the result of the excessive flattening of the brain in 

 this form. In the post-larval blenny, where no such flattening exists, 

 the fold (fig. 22 t.f.) is much shorter, and its limbs are much less 

 closely applied and thicker. Rabl - Rückhard's figures (7, Taf. VII, 

 figs. 17 — 19) make the double nature of the fold in Salmo very 

 apparent. 



The pineal body in the early larval stage is a roundish knob-like 

 structure projecting forwards from a constricted stalk. It is solid at 

 this stage (fig. 1 pn.), consisting of a single outer layer of small 

 roundish cells, and an inner fibrous area, interspersed with small cells. 



In the early post-larval stage the body is still solid, but con- 



