214 
changed his opinion. That part of a dorsal nerve may 
acquire a certain independence, may be wholly split off, 
as it were, from the main stem, is shown by the acusticus 
which is very generally considered as belonging to the 
facialis. That such a splitting occurred in the trigeminus 
may be easily accounted for by the fact that it innervates 
the prostomium as well as its own segment, and by the 
strong and special development of the first pair of visceral 
archs (the jaws) and the first pair of gill-slits (the mouth). 
Both the trigeminus and the facialis, the first two seg- 
mental nerves, as 1 am inclined to consider them, send strong 
sensory branches (rami ophthalmici trigemini et facíalis, 
ramus buccalis facialis) into the prostomium which, having 
no ganglion of its own, must be innervated from the 
anterior segments of the soma as far as its dermal sense-organs 
are concerned. This evidently has caused the strong devel- 
opment of these first two nerves and their ganglia, especi- 
ally of the trigeminus. In Amphioxus also we find (cf. fig. 26) 
that the first segmental nerve, in front of the first somite, 
has a double nature. 
Oculomotorius. — Finally we have the oculomotorius as 
the ventral root belonging to the praemandibular segment 
and innervating the eye-muscles that arise fromit. If it 
can be doubted whether the eye-muscles can be derived 
directly from segmental trunk muscles, then it also 
seems questionable whether the oculomotorius, trochlearis 
and abducens can be directly compared to the ventral 
roots of spinal nerves. In going from the trunk to the head 
in an embryo we see the ventral roots disappear in the 
Same way as the myotoms when we approach the auditory 
vesicle. A comparison with Petromyzon and Amphioxus, 
however, renders it probable that the myotomes and ventral 
roots of this region have atrophied only secondarily, 
and that at least the M. rectus externus with the N. abducens 
may be homologized to a regular myotome with its ventral 
root. Oculomotorius and trochlearis, however, afford more 
difficulties. 1 need only recall here the anomaly of the 
dorsal origin of the trochlearis, while the oculomotorius 
springs from the mid-brain which lies in front of the 
isthmus and accordingly does not belong to the epichordal 
part of the neural tube, the former stomodaeum. HATSCHEK 
(1892, p. 158), in Petromyzon, recognizes only the abducens 
as a segmental ventral nerve and derives the oculomotorius 
