MOTOR NUCLEI IN PHYLOGENY 481 
The viscero-motor column is situated in the midst of the 
undifferentiated formatio reticularis, through which must pass 
the majority of intrasegmental impulses, acting reflexly upon it 
from the overlying general cutaneous and associated areas. 
The presence of the long olfactory paths in the ventro-lateral 
area is more probably the result rather than the cause of the 
position of the viscero-motor column. 
The ventro-lateral displacement of the V—VII, as well as the 
X nucleus, from the primitive dorsal position has thus been 
in the direction of the greatest number of incoming impulses in 
accordance with the first concept of neurobiotaxis (vide supra). 
With regard to the close association of the motor nuclei of the 
trigeminus and facialis to form the rostral visceral motor column 
it is to be noted that these nuclei innervate musculature derived 
from the maxillary, mandibular and hyoid arches which is pri- 
marily concerned in the ingestion of food and not in respiration. 
During the process of feeding the musculature in question has no 
respiratory function whatever and respiration takes place at 
such times by ebb and flow of water through the gill clefts. 
The chief feeding reflexes in Bdellostoma are primarily initi- 
ated by tactile stimuli from the tentacles which are innervated 
exclusively by the trigeminal nerve (3, 100). 
The fusion (or perhaps the non-separation) of these two 
effector nuclei (V-VII) whose coordinate action is initiated by 
sunilar and almost simultaneous afferent impulses, may thus 
illustrate the second concept of neurobiotaxis (vide supra). 
A comparison of the total length of the caudal viscero-motor 
column in Petromyzon with that in Bdellostoma brings out the 
fact that the column in question is shorter in Bdellostoma than 
in Petromyzon by just the length of the IX nucleus. In other 
words, the vagus nucleus proper in Petromyzon is almost exactly 
the same length as the entire caudal viscero-motor column in 
Bdellostoma (fig. 8). 
The gap between the rostral and caudal viscero-motor column 
is smaller in Bdellostoma than in either Petromyzon or Myxine; 
while the caudal viscero-motor column overlaps the anterior 
horn (spino-occipital nucleus) to a considerable extent in 
