81 
vertebra !). Besides the prostomium it contains five segments, 
two pro-otic and three post-otic. No post-branchial segments, 
ie. segments behind the last gill-slit, have been incorporated 
into the skull, the occipital arch being situated over the last 
(fifth) gill-slit (Miss PLATT, 1898, p. 452). As a consequence of 
the restricted number of post-otic head segments the sphere of 
influence of the vagus extends beyond the cranio-vertebral limit 
and causes thefirst (in Anurans even the second) free spinal 
nerve to lose its dorsal root and ganglion. The vagus never sup- 
plies more than three gill-slits and as a consequence contains 
the rami post- and praetrematici of only two spinal nerves 
behind it, being the “spinalartiger Vagusanhang” and the 
first free spinal nerve, to which the dorsal root fails. 
The hypoglossus-musculature, according to Miss PLATT 
(1898, p. 452), is produced in Urodelans by ventral buds 
from the last epibranchial myotome (the third post-otic somite) 
and by two postbranchial ones (4t® and 5t® post-otic somites) 
and is innervated by the ventral roots belonging to the 
latter two, being the first two free spinal nerves “which 
constitute the wholly postcranial cervical plexus, or hypô- 
glossus. There is no epibranchial musculature in Amphibians. 
Head of Selachians. —In Selachians and Sauropsids, 
finally, the first processes of development are in a corres- 
ponding way influenced by the enormous accumulation of 
yolk in the egg. In both, the metencephalon, in contrast 
with both the foregoing groups, has strongly developed, and 
the hypophysis no longer originates in front of the mouth 
involution but from its roof. In both groups new segments 
have been added to the skull which we shall accordingly call 
aneocranium. Whereas in the glossopharyngeus-somite 
no myotome and in the primary vagus segment only a very 
rudimentary myotome develops, we find in Selachians 
behind the primary vagus some four well-developed myotomes 
belonging to the head region and instead of one neural 
arch the rudiments of four have been described for Acanthias 
by HOFFMANN (1894, p. 638), of which the anterior one 
has been compared by SEWERTZOFF (1895, p. 260) to the 
occipital arch of Amphibians. In front of it, just as in 
) If, at least, we do not consider the rudimentary “prae-occipital 
arch”, sometimes found in Urodelans in front of the “occipital arch”, 
and to which the occipital arch of Anurans, according to VAN SETERS 
(1921), corresponds, as another vertebra, 
LXXXII 6K 
