Observations upon the Development of the Teleostean Brain. 493 



first traces of the fold (velum) and the pineal evagination arise 

 simultaneously, the former immediately in front of the latter, Hoff- 

 mann shows that in Salmo the velum arises after the pineal body 

 has passed through its earliest stage, and at some little distance in 

 front of it. The same may be said of the Herring, only that the 

 velum arises considerably later than in Salmo. 



Balfour remarks (1, p. 400) that the roof of the thalamencephalon 

 becomes „very much shortened up by the approximation of the cerebral 

 rudiment to the mid-brain", and it may be assumed, I think, that 

 this shortening brings about the formation of the fold, or velum. 

 Thus the latter is, in Elasmobranchs, in the first instance due to the 

 upward rotation of the cerebrum, and this is equally the case in 

 Teleosteans. 



In later stages in the Herring (cf. | inch stage) the two limbs 

 of the fold (velum) are seen to be closely apposed to each other, the 

 whole projecting downwards and somewhat backwards as a keel-shaped 

 structure. 



As in the young Lepidosteus, the recess extends laterally over 

 the tips of the optic thalami (fig. 20), but its roof never acquires more 

 than a single layer of cells. The velum never extends back to the 

 same extent as in Salmo (cf. Hoffmann, 5, Taf. IV, Fig. 16) or even 

 Änarrhichas (cf. Me Intosh and Prince, 6, pi. XXIV, fig. 2). The 

 recess is well seen in a vertical longitudinal section (figs. 18 and 19 r. i.p) 

 of a 1-^i inch Herring. The superior commissure is very well marked, 

 lying (fig. 19 s. c.) at the point of origin of the roof from the pineal 

 stalk. 



Shipley's discovery of the superior commissure just in front of 

 the pineal body in the Ammocoete would appear to negative Balfour 

 and Parker's (3, p. 377) suggestion of a possible homology between 

 the thalamic vesicle in Lepidosteus, and the so-called pineal body of 

 Petromyzon. The commissure, when both structures are present, seems 

 to lie invariably behind the recess (or vesicle) as in Lepidosteus, 

 whereas, in Petroniyson, it lies in front of what Shipley considers 

 to be the pineal body; nor does the development of that structure, 

 as described by the latter observer, show any reason for supposing 

 that he has misinterpreted it. 



I have examined the infrapineal recess (figs. 21 and 22 r.i.p.) 

 in several larval and post-larval stages of Zoarces viviparus. The con- 

 dition is similar to that of the Herring (cf. fig. 19 r.i.p.)., but the 

 velum (/".) is deeper and does not extend backwards at all, although 



Zuol. Jahrb. IV. Abtli. f. Morph. gg 



