40 
of roots of the cervical plexus, the number of myotomes 
contributing to the formation of the hypobranchial muscula- 
ture must be very considerable, whereas there are not more 
gill-slits than in sharks. 
Agreement with the fin muscles. — The myotomes fol- 
lowing next produce in the same way muscle-buds which 
grow out into the rudiment of the fore-limbs and produce 
the musculature of the latter. Both sometimes form together a 
continuous series of myotomic buds, representing FRORIEP's 
(1885, p. 26) shoulder-tongue-string (Schulterzungenstrang), 
projecting externally as the shoulder-tongue-ridge, and in 
Selachians innervated by a single nerve-stem, the cervico- 
brachial plexus. This is composed of the ventral nerve-roots 
of the corresponding myotomes and only distally divides 
into two branches, a cervical one to the hypobranchial 
müsculature and a brachial one to the pectoral fin. In 
other Vertebrates the cervical branch is independent of the 
brachial plexus and represents the hypoglossus which 
ay be intra-cranial (Amniotes) or post-cranial (Petro- 
myzon, Amphibians). 
If with BALFOUR (1878, p. 102) we assume that the paired 
appendages are to be derived from a pair of longitudinal 
folds extending over the whole trunk, there is reason to 
suppose that formely a continuous series of ventral myo- 
tomic buds was present all over the trunk, as observed in 
some Selachians (Rajidae). Perhaps this might be compared 
with the growing out of myotomes into the metapleural 
folds in Amphioxus, which however extend also to above 
the gill-slits and are found even especially marked there. 
Then only secondarily the formation of myotomic buds 
would have ceased to occur in the region between the 
fore- and hindlimbs and in the branchial region, while be- 
tween the latter and the forelimbs the buds could have 
given rise to the hypobranchial musculature. 
owever, in Petromyzon we find the hypobranchial mus- 
culature well-developed, but no paired limbs. If the latter 
have not been secondarily lost, this circumstance does not 
support the above considerations. 
On the whole we get the impression from the statements, 
gathered above on different groups of Vertebrates, that the 
rule which also holds good for the appendages, viz. that the 
muscles are innervated by the ventral roots belonging to 
the myotomes from which they are derived, may equally 
