279 
oral fusion outlined in fig. 1” (reproduced here in fig. 19). 
Miss PLATT points to the circumstance that in Necturus 
the rudimentary hyomandibular pockets and the hyobranchial- 
clefts also fuse with one another ventraliy. The definitive- 
shape of the mouth is established by the elongation of the 
lateral extension of the oral fusion, while the longitudinal 
part of the latter becomes obliterated. 
The relation of the trigeminus to the mouth closely corres- 
ponds to that of the facialis to the spiraculum and that. 
of the glossopharyngeus and the vagus. 
to the following gill-slits. If indeed. 
the mandibular segment is to be consi- 
dered as the first segment of the soma, 
the above cited objection of ZIEGLER: 
against the derivation of the mouth. 
from two gill-clefts wholly looses it 
validity. It is especially the conclu- 
sions we shall reach in regard to: 
Fig. 19. Formation of Amphioxus (cf. anon) which plead for 
the mouth in Necturus the assumption that the gill-slits are 
aen Rn indeed older than the mouth and that 
PO the latter is to be derived from them. 
The nerve belonging to the first pair of gill-slits, repre-- 
sented by the mouth, would be then the trigeminus, which thus. 
ought to be considered as a single segmental nerve and not as: 
a double one, which would imply the falling out or failing of 
a pair of prae-oral gill-slits, of which in Craniates there is 
no indication. The scheme we arrive at in this way is still 
simpler than that of ZIEGLER and closely approaches that of” 
GEGENBAUR after he had discarded his view that the labial 
cartilages represent two praeoral visceral archs (1887, p. 79). 
The visceral archs appear now indeed to correspond to the- 
Segments incorporated into the head, the mandibular arch 
representing the first or trigeminus-segment, the hyoid arch 
the second or facialis-acusticus segment, the first branchial, 
arch the third or glossopharyngeus segment, etc. With 
HATSCHEK (1892, p. 159, 160) we reach the conclusion 
that the palaeocranium of Ammocoetes, into which the- 
glossopharyngeus and the vagus have not yet been incor-- 
porated, comprises only two segments, viz: the trigeminus- 
and the facialis-segment, besides the prostomium, which by 
HATSCHEK is not distinguished from the first segment, 
his “acromerite”. 
hed 
