A4S6 DAVIDSON BLACK 
of the surface of the cerebellum in sharks varies almost directly 
with the size of the organ in conformity with the general laws of 
cortical expansion enunciated by Baillarger and modified by 
Darests (cf. Kappers 68, 69, 71). 
Spino-occipital nuclei and roots (Nu. et rad. mot. Nn. spin. occ.). 
Only the first two rootlets of the spino-occipital series are charted 
in Selache and these probably represent separate fascicles of the 
first spino-occipital nerve (fig. 17C). They pass almost di- 
rectly ventrad from their dorsally placed nucleus of origin to the 
ventral periphery of the medulla (fig. 10). 
As in Bdellostoma, there is no line of demarcation between 
the anterior horn of the cord and the motor nuclei of the spino- 
occipital nerves. However, the rostral end of the anterior horn 
in Selache becomes thinned by the absence of ventrally placed 
motor cells, so that in the spino-occipital region the somatic 
motor column in this animal is characterized by a more dorsal 
position than in the succeeding cervical region. A similar ar- 
rangement of the spino-occipital nuclei and roots obtains in all 
selachians examined. The possible significance of these rela- 
tions in contrast to those obtaining in ganoids and teleosts will 
be discussed subsequently. 
The rostral limit of the motor spino-occipital nucleus in Selache 
is sharply marked and the reticular cells of the medulla are not 
arranged in such evident morphological continuity with this 
column as in Bdellostoma. The reticular cells form a nucleus 
whose elements are irregularly distributed in the formatio 
reticularis from a level somewhat rostrad of the entrance of the 
trigeminus, downwards to the calamus region. The largest 
cells tend to be situated centrally and the nucleus corresponds 
in general arrangement to the reticular nucleus of Raja de- 
scribed by van Hoevell (1. c.). The latter author recognizes 
three more or less distinct parts to this nucleus, viz., (a) nucleus 
reticularis inferior which is distributed on the level of entrance 
of vagus roots, (b), nucleus reticularis medius situated chiefly on 
the level of entrance of N.VIII, and (ce), nucleus reticularis su- 
perior distributed over the region rostral to the entrance level of 
N.VIII. 
