THE MUSCULAR SYSTEM 219 



belonging to the tenth myotome, which seems here far from 

 the fin, but even this eventually reaches the fin, as does also a 

 second one from the same myotome, forming buds XVIII and 

 XIX respectively. 



The cause of this apparent struggle to reach the fin on the 

 part of the most posterior myotomic buds, is one which ex- 

 plains also certain other characteristic features of the develop- 

 ment, and is found in the unequal rate of growth of -fin anlage 

 and of body axis, the latter considerably surpassing the former. 

 There results from this the concentration or bunching together 

 of the nerves of the free limb, especially noticeable in Fig. 57, 

 C, a circumstance favorable to the formation of a nerve plexus, 

 and as this concentration of a number of pairs of nerves to 

 form those supplying the limbs is also seen in the case of all 

 higher vertebrates, it is a convincing proof of the derivation of 

 the limb muscles from a more extensive series of myotomes 

 than that indicated by the adult size of the limb. 



From this sketch of the development of a limb as seen in the 

 selachians it becomes apparent that, were it possible in each 

 group of vertebrates to trace the derivation of each limb muscle 

 to a given myotomic bud, or, in other words, were it possible 

 to follow the later history of each separate myotomic bud to 

 the complex conditions of higher forms, a sure and certain 

 homology of the limb muscles could be carried out ; as a matter 

 of fact, however, the primitive history in the development of 

 limb muscles is found only in fishes, which, in their adult state, 

 are scarcely beyond the last of the three stages shown here, 

 while in all higher vertebrates, from the uro deles on, these 

 early stages are dropped out completely, and in a developing 

 limb, in which for a time the cells seem exactly alike, and with- 

 out differentiation of any kind, the first indication of any defi- 

 nite arrangement is the collection of these apparently indif- 

 ferent cells into masses that suggest the parts as they exist in 

 the adult. 



In such a case, then, the only recourse lies in the comparison 

 of adult forms, and here, owing to the complexity of the subject 

 and the technical difficulties in the way of such investigation, 



