MOTOR NUCLEI IN PHYLOGENY 479 
Bdellostoma and consider in how far the position of these nuclei 
presents evidence of the operation of neurobiotactic forces. 
Worthington has clearly shown that in Bdellostoma the two 
chief sources of information concerning its environment are 
respectively olfactory and tactile (100). 
The whole brain rostral to, and including, the so-called mid- 
brain is dominated, almost exclusively, by the olfactory appa- 
,ratus. On the other hand, the largest and most important 
tract of the hindbrain, one which dominates all the anatomical 
arrangement of the medulla and extends from the funicular 
nuclei to the level of the large trigeminal sensory root, is the 
general cutaneous system (99). Worthington remarks further 
that ‘‘this tract,’’ (ascending sensory trigeminal) ‘‘judging from 
Johnston (51), is relatively much larger in Bdellostoma”’ than 
it is in Petromyzon (I. ¢., p. 160). 
The fasciculus communis system in Bdellostoma, though defi- 
nitely developed, is small (Ayers and Worthington, 5), while 
the same may be said of the acustico-lateral system (Ayers and 
Worthington, 4). Both these, like the general cutaneous sys- 
tem, give rise to efferent fibers passing either by way of the 
fibrae arcuatae internae to the region of the heterolateral vis- 
cero-motor nuclei, or directly ventral to the homolateral motor 
column. 
Moreover these authors have shown that a free and extensive 
system of intercommunications between the general cutaneous, 
communis, and acustico-lateral areas by means of correlation 
neurones, is characteristically developed. In addition to these 
connections it would appear from the description and figures of 
these authors (4, p. 7 and 8, figs. 38, 40 and 43) that projection 
neurones also are related by their dendrites to each of the three 
areas mentioned. It follows from this that the secondary corre- 
lation tracts in the brain of Bdellostoma can not possess a very 
high degree of specificity of function. 
Thus, a condition exists in this form which is apparently not 
far removed from that obtaining in the medulla of the half- 
grown amphibian larva, where each correlation tract ‘‘may be 
actuated physiologically at the same time by two or more diverse 
physiological systems of the periphery” (Herrick and Coghill, 43). 
