470 DAVIDSON BLACK 
press here my sincere appreciation and thanks to Miss de 
Lange for the skilled and careful manner in which she carried out 
this important part of the work. 
CYCLOSTOMI 
Motor nuclei in Bdellostoma dombeyt 
This form is the common hagfish of the American Pacific 
coast which, according to Worthington, is identical with the 
Californian variety not infrequently described as B. stouti (100). 
Several brains of this form prepared by different methods and 
cut in both transverse and sagittal series were studied. The 
reconstruction chart (fig. 7) was prepared from one series cut 
transversely and stained by the method of Cajal. 
Spino-occipital nuclei and roots (Nu. et rad. mot. Nn. spin. occ.) 
At the junction of the cord and medulla in Bdellostoma certain 
very definitely specialized nerves may be recognized, each 
possessing one sensory and two motor roots. The essential 
difference between these specialized nerves and the cervical 
motor roots lies in the total absence in the former of peripheral 
branches supplying dorsal trunk musculature (Worthington, 
99). Furbringer’s term ‘spino-occipital’ (24) has been adopted 
by Worthington to describe these nerves and this name has 
been retained in the present description.’ 
Both the motor roots of the first spino-occipital nerve have 
their superficial origin slightly caudad of the level of entrance 
of the sensory root, while the reverse is the case in the second 
spino-occipital nerve. In the latter nerve, four motor rootlets 
uniting to form two main stems were described by Worthington 
(99) but I was unable to confirm this observation. 
’ However, it is evident that the nerves in question are derived from segments 
more rostrally placed than those from which the first two spinooccipital roots 
arise in selachians (Neal, 84, Furbringer, 25, et. al). Thus, both here and in the 
subsequent description of this region in selachians and ganoids, the term ‘spino- 
occipital’ is used in its broadest collective sense to designate certain precervical 
motor elements whose number and segmental relationships are subject to con- 
siderable variation in these different groups. 
