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of ganglia fused to form the vagus-ganglion appears not 
to correspond to the number of gill-arches innervated by 
the ventral branches of the vavus, being four in pentanch 
Elasmobranchs. According to GUTHKE (1906), the complex 
vagus ganglion sends three roots into the medulla and also 
three branches into the last three inter branchial gill-archs. 
Thus ZIEGLER (1908) and his disciples consider the vagus 
ganglion as formed by the fusion of three ganglia, the ganglion 
of the fourth branch, which runs behind the last gill-slit, 
(cf. fig. 17) or of the last branches in hexanch and Eeptanch 
forms, having been lost and these branches themselves having 
united with the fore-going, so that the third or last branch coming 
from the ganglion sends nerves to two, three or four gill-slits. 
VAN WYHE, who originally (1882) considered the vagus as a 
quadruple nerve, afterwards (1889, p. 562), on the discovery of 
rudimentary dorsal ganglia to the last two head-myotomes 
(VAN WYHE, 1886, OSTROUMOFF, 1889), recognizes only two 
vagus-roots. The results of embryological and anatomical 
researches on Petromyzon point to the conception, advocated 
by HATSCHEK (1892, p. 157), that the primary vagus is here a 
single nerve only, which in Gnathostomes probably fuses with 
one spinal nerve and its ganglion, which are still independent 
in Petromyzon, and further “collects” the ventral part (rami 
prae- and posttrernatici) of the dorsal roots of as many 
subsequent spinal nerves as there are gill-slits supplied 
by it (“partial polymerism” of the vagus, HATSCHEK, 1892, 
b.io2 us the ramus branchio-intestinalis of the vagus 
would be a collector in the same way as the ramus lateralis 
of that nerve according to EISIG and others. 
The main ganglion of the trigeminus lies over the man- 
dibular arch, and the first ganglion of the trigeminus, 
the ganglion ciliare with the ramus ophthalmicus profundus, 
is considered to belong to the praemandibular segment. 
Thus the trigeminus is considered by VAN WYHE and 
ZIEGLER to represent a double segmental nerve. 
Neural crest of head and trunk. — Another question 
which has occasioned controversy is, whether the neural 
crest, which gives rise to the centrogenetic part of the 
head ganglia, is to be considered as the direct conti- 
nuation of the neural crest of the trunk. In fact there is 
this difference between the spinal and the head ganglia, 
besides the mixed character of the latter, that the former 
are found on the inner side of the somites, the latter on the 
