MOTOR NUCLEI IN PHYLOGENY 501 
in sharks is much more rostrally placed and in all the latter 
forms lies in a dorsal position in the Sylvian gray immediately 
caudal to the oculomotor nucleus. 
In Petromyzon the trochlear nucleus lies on the exit level of 
the motor V root and caudad of its own root exit, while among 
sharks this nucleus is always placed far rostrad of the exit level 
of the motor V root and with few exceptions (e.g., Selache, 
Hexanchus) among these forms lies wholly rostrad of its own 
root exit. 
The position of the trochlear nucleus just caudad of the oculo- 
motor nucleus appears to be characteristic of all vertebrates 
above cyclostomes in which the eyes are functionally well de- 
veloped. Thus the close mutual association of these two nuclei 
appears to have occurred phylogenetically at the same time as 
the development of the midbrain roof to form the great terminal 
nucleus for the optic tract. 
On the other hand, the site at which the trochlear decussation 
and exit occurs seems to be quite variable even among closely 
allied forms (cf. teleosts) and it would appear that the root of 
this nerve, unlike its nucleus, is subject to variable influences 
of a purely mechanical character. 
In conclusion, it is evident that the motor roots and nuclei in 
Selache maxima are arranged to form a pattern which is in all 
essentials similar to that obtaining elsewhere among selachians. 
In certain respects this selachian pattern exhibits primitive 
characters, i.e. (a), the emergence of the motor roots in phylo- 
genetic sequence, viz, LIT, 1V, V, VIL, VI,.EX, X, sp. ece-.; (b), 
the predominantly dorsal situation of both visceral and somatic 
motor nuclei; (c) the direct continuity and dorsal position of 
vagus and accessory nuclei, etc. In other respects unmistak- 
able signs of specialization are evident, such, for example, as 
the relations of the rostral end of the spino-occipital nucleus, the 
caudad development of the accessory complex, the development 
of the ascending motor VII and IX roots and the rostral position 
of the trochlear nucleus in direct contiguity with that of the 
oculomotor nerve. 
