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NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 255 
b. RELATIONS or ENORPHALOMERR VI. 
The present structure and relations of the component parts of what I 
regard as the primitive sixth cephalic segment have been considerably 
changed comogenetically by the development of the otic capsule, Aris- 
ing from what in all probability was primitively a sensor organ of the 
dorsal lateral line (Ayers), the great enlargement and subsequent invagi- 
nation of this capsule bring about ontogenetically the degeneration of 
the musculature of the 5th somite, whose cells, after assuming the 
elongated spindle form of embryonic muscle cells, are transformed in 
early stages into loose mesenchyma. In Ammocovtes, however, only the 
median portion of the Ist post-otic somite disappears during ontogeny, 
while the lateral portion forms the most anterior segment of the lat- 
eral body musculature (muse. lateralis capitis anterior, von Kupffer). 
Furthermore, in Squalus the development of the otic capsule causes a 
shifting backward of the point of exit of the fibres of the glossopharyn- 
gous, whose ganglion cells were proliferated from encephalomere VI; 
moreover, the fibres of this nerve may be traced in the neural tube as far 
forward as encephalomere VI, in which, it is my opinion, their nuclei lie. 
The growing ganglionic Anlage of this nerve meets the mesoderm between 
the 4th and 5th somites (Fig. 13), and I assume that it was primitively 
related, as are the dorsal nerves of Amphioxus, to a myoseptum, i. e. the 
one primitively between somites 4 and 5. The sensor fibres of this 
nerve innervate the skin of the present 2d visceral cleft (Fig. 14), 
which was, I assume, primitively inter-somitic in position and situated 
ventral to the myoseptum between the 4th and 5th somites. Its motor 
fibres innervate the splanchnic musculature of the present 3d visceral 
arch, probably a primitive relation. The abducens nerve, I believe, 
represents the primitive ventral nerve of this metamere, 
c. RELATIONS Or ENCEPHALOMERE V. 
The fourth somite, the one corresponding to the fifth cephalic segment, 
is the most rudimentary of all the cephalic somites. The phylogenetic 
loss of its musculature and the ontogenetic dissolution of its cells into a 
loose mesenchyma may be explained as due to the same cause as that 
operative in the case of the 5th somite, the development of the otic cap- 
sule. The dorsal nerve of this segment, the facialis, is inter-somitic in 
position, occupying the constriction dividing the 3d and 4th somites 
(Figs. 11-17), and its motor fibres innervate the (splanchnic) muscu- 
lature of the corresponding (2d visceral or hyoid) arch, Correlated with 
