283 
has become emancipated from the main stem as the acus- 
ticus, which is generally considered as a branch of the 
facialis. The gill-slit belonging to this segment shows from 
the beginning differences to the other gill-slits. In Elas- 
mobranchs it is the spiracle, in Cyclostomes, Amphibians 
and Teleostei it does not open to the exterior; while in 
terrestrial Vertebrates the auditory vesicle enters into close 
connection with its rudiment which widens to the tympanic 
cavity in which, in Mammals, ossified parts detached from 
the first or mandibular arch, the malleus and incus, transfer 
the vibrations from the tympanic membrane to the labyrinth. 
Backward extension of the skull. — A very difficult problem 
which has given rise to much controversy is that of the 
backward extension of the skull, in which especially the 
appreciation of the spino-occipital nerves (FüRBRINGER) 
has played an important role in later years. 
According to the views of GEGENBAUR and VAN WYHE the 
number of segments incorporated in the vertebral part of the 
skull is indicated by that of the visceral arches. The last 
branchial bar, e.g in Scyllium and Pristiurus on which 
VAN WYHE worked, corresponds to the last segment of the 
neurocranium and, since this fifth branchial arch lies behind 
the last gill-slit, there is only one post-branchial segment 
incorporated into the skull, while the others are epibranchial. 
According to VAN WYHE (1882) there are in these forms nine 
head segments, a number which is reduced to seven, the number 
of the visceral arches, if we do not recognize the praeman- 
dibular cavity as a somite and reject the idea of a double 
nature of the hyoid-segment, for which, as has been recently 
emphasized by GOODRICH (1918, p. 5), no serious evidence 
has ever been found. Of these seven segments there are 
two pro-otic, four post-otic and epibranchial and one 
post-otic and post-branchial. 
Ventral occipital nerves. — In Elasmobranchs a number of 
ventral roots leave the skull in the occipital region, under and 
behind the vagus. Formerly, according to GEGENBAUR's (1871, 
p. 530) example, they were generally compared to the hypo- 
glossus of Amniotes, and GEGENBAUR (1871, p. 521, 1872 
p. 268) who considered the vagus as a quadrivalent dorsal 
nerve originally called them the ventral vagus roots. Indeed, 
as shown first by VAN WYHE (1882, p. 36), their situation 
in ontogeny corresponds to that of the occipital somites, 
generally considered as the region of the vagus, and so do 
