478 DAVIDSON BLACK 
Discussion 
On comparing the reconstruction of the motor nuclei in 
Bdellostoma dombeyi with that of the nuclei in Petromyzon 
fluviatilis (fig. 7), two striking differences are at once apparent: 
firstly, the ventral position of all motor nuclei in Bdellostoma, 
and secondly, the complete absence in the latter form of the 
oculomotor, trochlear, and abducens nerves. In these two 
respects Bdellostoma closely agrees with Myxine glutinosa (vide 
Eds ES). 
In all these forms there is a significant division of the visceral 
motor nuclei into two parts: (a), a caudal portion, consisting of 
the vagus and glossopharyngeus nuclei in Petromyzon, and of 
the vagus nucleus alone in Bdellostoma and Myxine; and (b) a 
frontal portion, consisting of the V-VIT nucleus. 
It is important to note that the position of the last named 
nucleus with reference to the level of exit of the V and VII 
motor roots, is almost exactly similar in each of the three forms. 
It is strongly probable that in phylogeny all motor nuclei 
tend to be situated originally on the segmental level of their 
emergent roots and close to, or within the central gray. Kap- 
pers has already drawn special attention to this point. Thus, in 
Petromyzon the position of the chief bulk of the motor VII 
nucleus in the central gray, approximately on the exit level 
of its motor root, is indicative of a primitive condition. The 
fusion of the V-VII motor nuclei (or it may be, their non-sepa- 
ration) Kappers also considers somewhat primitive for various 
reasons, though the possibility of such a condition is to a certain 
extent no doubt dependent upon the absence of a large gusta- 
tory VII-IX—X center in these forms (see especially 61 and 64). 
The ventro-lateral displacement of the motor V—VII nucleus 
in Myxine has occurred under the dominating influence of the 
chief reflex paths (especially the general cutaneous system) 
situated in the lateral and ventral area of the medulla (R6thig, 
87, Réthig and Kappers, 88). 
It will be of interest then to inquire into the arrangement of 
the chief reflex connections of the motor V and VII nuclei in 
