198 HISTORY OF THE HUMAN BODY 



through an active larval period, and hence exhibits the earlier 

 stages in functional activity and consequently in greater com- 

 pleteness. 



From both this source and from the study of adult com- 

 parative anatomy certain principles may be deduced concern- 

 ing the formation of muscles, some of which may be noted 

 here. 



I. Separation of an indifferent mass into several elements. 

 This may be done in several ways : 



(a) By the growth of process from the surrounding skeletal 

 parts, thus furnishing separate points of origin for different 

 bundles of fibers. 



(b) By the growth of a skeletal process across the fibers of 

 a long, band-like muscle, thus cutting it in two; a secondary 

 segmentation. 



(c) By the higher specialization of the parts of insertion, 

 thus causing a splitting up of the muscle bundle. 



Illustrations of the first of these may be seen in the develop- 

 ment of the processes on the vertebrae, which results in the 

 breaking up of the indifferent myotomes into the extremely 

 complex slips seen in the muscles of the vertebral column of 

 higher forms. The second method is rare, and occurs in the 

 case of one of the hyoid muscles of the frog (the fourth 

 petro-hyoideus) , and in two of the occipito-cervical muscles 

 in the mammals (obliqui capitis, cf. Fig. 55). The third case 

 finds a complete illustration in the differentiation of the digits 

 in mammals and the splitting up, both of the tendons and of 

 the bellies of the common flexors and extensors, in exact ac- 

 cordance with the degree of independent action of the separate 

 digits. 



II. Fusion of separate elements. 



This may occur in the case of the degeneration of a function, 

 but it is seen in a progressive instance in the case of long 

 muscles derived from elements taken from separate myotomes. 

 Thus the external oblique muscle of the abdomen is a meta- 

 meric muscle formed by contributions from a series of suc- 

 cessive myotomes. In salamanders this is barely differentiated 



