200 HISTORY OF THE HUMAN BODY 



Both phylogenetically and ontogenetically the axial muscles 

 are the first to appear, and forcibly suggest some lost ancestor 

 in the form of a segmented worm, the muscles of which were 

 mainly longitudinal in direction and repeated themselves meta- 

 merically. The appearance of these, in the form of meso- 

 dermic somites, is one of the earliest post-gastrular stages in 

 the embryo, and is the first suggestion of segmentation. In 

 the lower vertebrates the muscle somites develop through the 

 longer process of the formation and extension of pairs of 

 lateral diverticula and the subsequent separation of the epi- 

 meres, as in the theoretical sketch given above [Chap. Ill], 

 but in the Sauropsida and Mammalia, through acceleration of 

 development, the epimeres.are separated from an indifferent 

 mesoderm in the form of approximately cubical blocks, ar- 

 ranged in pairs on each side of the nerve-cord and notochord, 

 and were long considered to represent " primordial vertebrae," 

 a name occasionally used even at the present time, although 

 its literal meaning has been long since discarded. From these 

 the myotomes develop through the formation of longitudinal 

 fibers, and the mesenchyma of the intervals between the meso- 

 dermic somites becomes transformed into the myocommata, 

 to which, on either side, the muscle fibers are attached. 



There is thus formed the primitive system of axial muscles, 

 a condition which is retained with but little modification in 

 the primarily aquatic vertebrates, that is, in Amphioxus, cy- 

 clostomes, fishes and tailed amphibians. In the first of these 

 the myocommata are not in the form of planes, but are bent in 

 the middle at an acute angle, the point directed forwards, and 

 are set into one another so that several consecutive myocom- 

 mata would be cut in a cross-section through the body at any 

 plane. In many fishes the myocommata become still more com- 

 plex and consist of several pairs of cones, those of successive 

 myocommata being set into one another like nested cups. This 

 explains at once the shortness of fiber of fish meat, and also its 

 curious division into sets of concentric circles, when cut in 

 cross section, two phenomena with which everyone is familiar. 

 In the urodelous amphibia the myocommata are almost planes, 



