84 
the epibranchial musculature is formed (DOHRN, 1885, p. 446, 
HOFFMANN, 1898, p. 265) which in all other Gnathostomes, 
and also in rays already, is absent. The greatest number 
of epibranchial myotomes will be found in hexanch and 
heptanch Selachians where the number of gill-slits is greatest. 
Here indeed the epibranchial musculature is best developed 
(FüRBRINGER, 1897, p. 416) andso are the occipital nerves 
supplying them which lie in front of the hypoglossus-roots 
(v, w‚ Xx, according to FüRBRINGER). 
In the development of the somites from the unsegmented 
mesoderm a retardation in the head region, correlated with 
the degree of degeneration of the corresponding myotomes, 
is to be noticed, The first somite to be differentiated ís 
as a rule the first to develop a permanent myotome. This 
serves to explain why in the Craniata the development of 
the somites begins in the neck region and not, as in Amphioxus, 
with the foremost one (NEAL, 1898, p 195). Thus in Acanthias 
the 3rd or 4tk post-otic somite is the first to develop. As 
we have seen, this rule has been confirmed for Petromyzon 
by KOLTZOFF (1902, p. 318). Here the first post-otic somite 
is the first to appear. 
The hypobranchial musculature (Musculi coraco-arcuales) 
is formed in Selachiï from the ventral buds of a number 
of myotomes behind the last gill-slit. To judge from the 
number of ventral roots supplying it (cervical plexus, hypo- 
glossus, cf. FüRBRINGER, 1897, p. 404), the number of 
myotòmes contributing to the formation of this musculature 
is greater in rays where some seven or eight ventral roots 
participate in its innervation. In sharks this number is 48 
a rule from four to six of which none, one or two may be 
number of vertebral elements is assimilated by the cranium 
that the whole brachial plexus, consisting of ventral and 
