106 THE ANCESTRY OF VERTEBRATES 



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 '.'ot 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 myotomjc 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 

 musculature and a brachial one to the pectoral fin. In 

 other Vertebrates the cervical branch is independent of the 

 brachial plexus and represents the hypot^lossus which 

 may 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 intp 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. 



However, 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 

 suppoit 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 



