270 
oblique backward course, this incongruity appears to be 
much less distinct in very young stages where the gill-slits. 
have just broken through. It is precisely the oblique course 
of the posterior head nerves, as argued already by. 
GEGENBAUR (1872, p. 253, 275), which is a strong indi- 
cation of the original congruence between the cranium and. 
the series of gill-slits, however much the two may differ 
in backward extension in the adult. The curve in backward. 
direction made by the ventral nerves of the cervico-brachial 
plexus round the last gill-slits to reach the ventral side of 
e body, must be considered equally as a result of the 
secondary backward expansion of the series of gill-slits, 
which in young embryos appears to be still much less 
pronounced. We come then to the conclusion that GEGENBAUR: 
was not far from the truth in assuming that the posterior 
limit of the Elasmobranch cranium coincides with the back- 
ward extension of the gill-slits, though, as we shall see further 
on, we may not go so far as to assume that the number of 
segments incorporated into the cranium may be deduced: 
from the number of visceral archs. 
As to the second part of the third question it need not 
again be emphasized that the branchial region does not 
belong to the primarily unsegmented anterior part of the 
head, the prostomium, but to the segmented soma. 
Thus on the whole my theory leads to a confirmation 
of the views of GEGENBAUR, VAN WYHE and ZIEGLER 
with his disciples, and to a rejection of those of FRORIEP- 
in so far as his conception of the branchial region as an 
unsegmented head region is concerned. 
Significance of praemandibular cavity. — As we have seen, 
the praemandibular segment, at first described by BALFOUR 
(1878), is considered by VAN WYHE and ZIEGLER as the 
first segment of the soma. To this end the N. trigeminùús 
must be considered as a double nerve, the ramus ophthal- 
micus profundus with the ciliary ganglion representing the 
dorsal root to the first segment, while the oculomotorius- 
is taken as the ventral root. As a matter of fact the latter 
innervates exactly those four eye-muscles which are produ- 
ced by the praemandibular segment, which lies closely 
applied to the eye-ball. Thus everything required for à 
segment is present with the exception, however, of one 
thing: the division into a dorsal and a ventral part. Only 
the former is present, the latter is missing, and with it the: 
