87 
somewhat less developed than the spinal ganglia behind 
it, though not so rudimentary as the ganglia in front of it. 
Neither in Amniotes nor in Acanthias does it produce a 
regular dorsal nerve. 
It will be better, however, to leave now FüRBRINGER's 
nomenclature and to replace it by the numbering of the 
segments shown in plate I. 
The pro-otic and the anterior post-otic somites are no 
longer to be recognized even in the earliest stages of 
development. The first myotome probably develops from 
what we may consider as the third post-otic somite. The 
auditory vesicle acquires certain relations to the first of the 
gillpouches behind the mouth. 
General conclusions. — From the foregoing consider- 
ations the following conclusions result. Though in such 
Selachians as Scyllium and Pristiurus the number of head seg- 
ments happens to correspond with that of the visceral archs, 
we must yet conclude, in opposition to GEGENBAUR (1872), 
that on the whole there is no relation between them. The 
number of gill-slits may be considerably greater than that 
of the segments of the skull, as is the case in Petromyzon, 
or the skull may reach just as far as the last gill-slit, as in Uro- 
delan Amphibia, or a greater or lesser number of post-branchial 
segments may belong to it, as in Selachians and Amniotes. 
In the first case the roots of the hypoglossus lie far behind 
the skull, in the second case it is formed from the anterior 
free ventral roots, in the third case a greater or lesser 
number of hypoglossus- roots become intracranial or occipital. 
In the first case we shall speak of an incomplete palaeocranium, 
in the second case of a complete palaeocranium, in the 
third case of a neocranium. It depends on the number of 
gill-slits whether the hypoglossus roots will lie far behind 
e vagus or within the reach of its sphere of influence 
Which causes the dorsal ganglia behind it to atrophy. 
That originally the hypoglossus has nothing to do with the 
vagus is shown by Petromyzon. When the number of gill- 
slits decreases, the hypoglossus shifts forwards, i. e. its roots 
now belong to segments more in front. This does not mean 
to say, however, as FüRBRINGER (1897, p. 440) assumed, 
that the myotomes and their ventral roots themselves shift 
forwards. Of such a “stetiges Vorrücken” there is no 
question. The roots of the hypoglossus may by the process 
mentioned above come so close to and under the partially 
