iv MYOTOMES 205 



next the endoderm representing the sclerotome. Cells proliferat- 

 ing from this invade the myocoele and completely fill it up. It 

 is only in later stages that the myotome becomes extended into 

 the normal plate-like form by active growth at its inner (dorsal) 

 and outer edges. Of the two walls of this stage the inner admittedly 

 becomes converted into muscle -cylinders. The outer becomes 

 loosened out into a mass of irregularly shaped cells and these are 

 commonly believed to give rise to dermis. Iu view of what happens 

 in Lepidosiren, where accuracy of observation is so much more easily 

 attained to, it seems advisable not to accept this as absolutely 

 certain. 



At the same time it may be allowed that there is no a priori 

 difficulty in the way of admitting that portions of myotome which in 

 one type of Vertebrate give rise to muscle, may in another have 

 ceased to do so, for, as already indicated, a quite similar process of 

 concentration of muscle -development in a localized portion of 

 somatic mesoderm is a fundamental characteristic of the whole 

 Vertebrate phylum. 



The series of paired myotonies, each composed of a mass of 

 longitudinal muscle-fibres traversing it from end to end, forms the 

 material out of which is formed the, often extremely complicated, 

 system of voluntary muscles of the adult Vertebrate. The various 

 myotomes as they increase in size become divided up into it may be 

 numerous pieces and these are pushed hither and thither by processes 

 of differential growth until the arrangement of the numerous adult 

 muscles contrasts greatly with the simple longitudinal arrangement 

 of the original myotomes. During the various displacements which 

 it undergoes the individual muscle or fragment of myotome remains 

 in organic connexion with its nerve-centre by means of its motor 

 nerve and the course of these nerves in the adult frequently gives 

 an important clue to the developmental migrations of the particular 

 muscles. 



No attempt will be made here to follow out the evolution of the 

 complicated muscular arrangements of the adult beyond a short 

 sketch of the method in which the muscles of the fins or limbs 

 originate. 



The median fin is simply the extension of the body in the median 

 plane and we should therefore naturally expect it to be muscularized 

 by prolongations of the myotomes growing into it. The actual 

 process is clearly illustrated in Fig. 113. In A a muscle-bud is seen 

 to be projecting from the end of each myotome where a median 

 fin is developing — the upper group of buds belonging to the dorsal 

 fin, the lower to the anal. The buds diminish in size towards each 

 end of the series and in the case of the dorsal fin, towards its 

 anterior end, there are a considerable number of abortive buds 

 which never come to anything. The muscle-buds grow into the fin 

 fold and then become cut off from the main part of the myotome to 

 form the muscles of the fin as is shown in B. 



