dat? GEORGE W. BARTELMEZ 
recognized hitherto, for it is not clear except in larval brains. 
In all fishes certain cells of the nucleus have enlarged and other- 
wise differentiated as a result of direct connections with the pri- 
mary sensory centers of the bulb. This connection has short-cir- 
cuited the primitive reflex path between the sensory centers and 
the motor nuclei of bulb and cord. The sensory connections of 
the nucleus have been emphasized by Cajal and others. The 
most important path so developed was between the acoustico- 
lateral centers and the motor nuclei of the muscles used in swim- 
ming. In the region of the acoustic centers we find highly dif- 
ferentiated cells of the nucleus motorius tegmenti arranged in 
pairs on either side of the mid-line, close under the acoustico- 
lateral decussation, whither they have presumably migrated in 
response to the close association with the acoustic system (Kap- 
per’s neurobiotaxis). These great cells are probably homologous 
with the Miiller’s cells of cyclostomes and they agree with them 
further in that the axone does not usually cross at the level of 
the cell body. 
One pair of such cells has established a direct connection with 
the vestibular root fibers by means of an enormous lateral den- 
drite and a peculiar synapse termed the axone cap. They are 
the cells of Mauthner and are among the largest and most highly 
differentiated nerve cells we know. ‘This view of the origin of 
Mauthner’s cell is based upon the following facts. In the cyclo- 
stomes there is a pair of enlarged Miiller cells at the anterior 
VIIIth level in exactly the same position as the teleostean Mauth- 
ner’s cells, differing from the latter chiefly in that the axones are 
uncrossed. In as much as we find in the teleostean Miller cells 
all transitions from an uncrossed to a crossed condition (p. 99), 
this is not a particularly significant difference. The Miller cells 
of Ameiurus show transitions from the large cells of the nucleus 
motorius tegmenti to the Mauthner cells in other respects such 
as internal structure. Furthermore, Mauthner’s cell receives all 
of the impulses which are common to every part of the nucleus 
since its ventral dendritic field is identical with that of the asso- 
ciated groups of the nucleus. Finally, the axone of Mauthner’s 
