38 
ventral side. Thus the whole hypoglossus is post-cranial 
and begins far behind the skull. Now a comparison of embryos 
of different lengths shows that the postotic myotomes 7— 12, 
which are originally post-branchial, come to lie ultimately 
above the hinder part of the branchial basket, í. e. become 
epibranchial, by the backward extension of the branchial 
basket. By the same process, as NEAL was able to observe, 
the ventral roots of these myotomes, passing originally free 
behind the last gill-slit, unite one by one as the branchial 
basket elongates and thus form the hypoglossus. The hypo- 
branchial musculature in Petromyzom shows a secondary (?) 
segmentation corresponding to the situation of the gill-slits 
but not to that of the epibranchial muscles to which myotomes 
of post-branchial origin have been added (Fig. 20). 
As will be pointed out later, there is reason to consider 
the Amphibians next after Petromyzon and before the 
Selachians. We learn from the observations of Miss 
PLATT (1898, p. 452) that here ventral buds from three 
myotomes, the last epibranchial (3rd post-otic somite) and 
the first two post-branchial — 4tt and 5t® post-otic somites, 
here at the same time the first and second post-cranial, 
as we shall see afterwards — produce the hypobranchial 
musculature. This is innervated by a post-cranial hypo- 
glossus-nerve composed of the ventral roots of the latter 
two somites. 
In Selachians, as may be concluded from the statements 
of NEAL (1897, p. 450) and HOFFMANN (1898, p. 263), the 
hypobranchial muscles are also produced exclusively or 
nearly exclusively by post-branchial myotomes. Buds from 
five myotomes representing in Acanthias, according to NEAL, 
the 4fh— 8th post-otic somite, form the hypobranchial 
musculature. Since the fifth post-otic somite, according to 
the conception of VAN WYHE and his followers, is here the 
first post-branchial one, there is again only one epibran- 
chial myotome that contributes to the formation of the hypo- 
branchial musculature, while the others are post-branchial. 
According to HOFFMANN’s (1898, p. 261, 263) statements 
it is even exclusively from the post-branchial myotomes 
that in Acanthias the hypoglossus musculature is produced, 
that of the 4tt post-otic somite not contributing to its 
formation. This musculature according to NEAL is innervated 
again by ventral roots belonging to the same myotomes 
from which it has originated, viz. those corresponding to 
