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the lateral pait of the dorsal branches (ramus lateralis) and 
further the lateral and visceral part of the ventral branches 
{rami post- and praetrematici) of the dorsal spinal nerves 
behind it Evidently under the influence of the vagus the 
anterior spinal ganglia behind it are not so well developed 
in Gnathostomes as those fursher back. In none of the 
publications on the Cyclostomes is any process of this 
nature referred to. The first spinal ganglion behind that 
of the primary vagus fuses with it (“spinalartiger Vagus- 
anhang“, HATSCHEK, 1892, p. 156). 
For the first time we find in Pefromyzon a cartilaginous 
skull, comparable to the head cartilage of Cephalopods and 
arising, like the latter, in close connection to the central 
part of the nervous system and the main sense-organs. lts 
extension is less than in higher Craniates and comprises, 
besides the prostomium, only two segments (HATSCHEK, 
1892, p. 159). This conception results from the observation 
that the cartilaginous skeleton does not extend further back 
than the auditory capsule and that the ganglion of the 
-glossopharyngeus and the vagus is situated behind it. 
However, the considerable size of the auditory capsule 
causes the latter to extend backwards into the anterior 
two post-otic segments in which the internal parts 
of the myotomes as a consequence atrophy, while their 
myocommata are attached to the auditory capsule (cf. fig. 
29). The first neural arch is the one situated in the myo- 
comma between the third and the fourth post-otic somite 
(fig 29, oec. a). Membranous walls connect the cartila- 
ginous skull with the first neural arch. Through this 
membranous part of the skull the glossopharyngeus and 
the vagus pass. 
We shall call a skull as found in Petromyzon an in- 
complete (partly membranous) palaeocranium. 
Finally it may be observed that the hypophysis originates 
from the ectoderm in front of the mouth involution. 
Amphibian head. —Next to the Cyclostomes to which 
they show a close resemblance in their earliest development 
come the Amphibians in which the hypophysis still originateS 
in front of the mouth involution, while this group stands 
also nearest to Petromyzon in the structure of the brain, 
especially of the metencephalon. The roof and the hinder, 
-membranous, part of the Cyclostome skull, between the 
anterior, cartilaginous, region and the first neural arch, has 
