( 898 ) 
In the trochlear nucleus it is conspicuous that this les very 
dorsally, above the aquaeduct in the velum (Fig. 8), as has been 
described by Scunne *) and observed by TrersaKorr *) in Ammocoetes. 
The further topography of this nucleus shows that it lies nearer 
the trigeminus root and closer to the trigeminus nucleus than to the 
nucleus of the III nerve. Moreover the trochlear nucleus lies 7 toto 
behind its root-end (Fig. 1 and Trersaxorr |. ¢.). 
Ht RY 
Fig. 2. Selache. 
If we compare these relations with Fig. 2, which represents the 
topographic relations in Selache, we immediately find a difference 
in the position of the oculomotor nucleus, for the nucleus lies consi- 
derably farther frontally than in Petromyzon and surpasses the frontal 
boundary of its root (see also Seyllium, fig. 9). A ventral III nucleus 
does not appear here, all the cells lie in the upper third part of 
the mid-brain basis (fig. 10). Still greater are the topographical 
differences shown by the trochlear-nucleus and its root in com- 
parison with Petromyzon. The trochlear nucleus no more lies 
dorsally from the aquaeduct im the velum, but to the side of the 
aquaeduct, practically under it, (fig 2). The nucleus is larger than 
in Petromyzon. A part of it still lies behind the root-entrance, 
another, much larger part lies in front. ®) 
The distance from the IV nucleus to the V root is greatly 
enlarged, and the shifting towards the Ili root is so pronounced 
that the II] and IV nuclei partly overlap each other or pass into 
each other (Figs 2 and 9). 
1) Scumune: Das Gehirn von Petromyzon fluviatilis. Abhandl. der Senckenber- 
gischen Naturforschenden Gesellschaft, vol. 30 p. 441 1907. 
2) Trersakorr: Das Centralnervensystem von Ammocoet-s. IL. Das Gehirn. Archiv. 
f. Mikrosk. Anatomie vol. 74 p. 713 1909. 
3) The topography of the IVth root and nucleus is not the same in all Selachii. 
Here I take Selache as object of demonstration because it seems more fit for com- 
parison than Scyllium. Moreover our preparations of Scyllium did not allow us to 
fix the limits with so much certainty as those of Selache. (Added in the English 
translation). 
