THE MUSCULAR SYSTEM 217 



head as well as the trunk possessed a series of successive my- 

 otomes, that certain of those furnished materials from which 

 the eyeball muscles were derived and that all other muscular 

 elements thus disappeared at a very early period. [Cf. Fig. 

 126, A, and the text accompanying it.] 



The appendicular muscles are the direct derivatives of cer- 

 tain axial metameres, as is especially well shown by the 

 ontogenetic repetition seen in selachian embryos. Here the 

 first indication of a lateral fin is in the form of a mass of tissue, 

 appearing externally as a slight, shelf-like projection, flattened 

 dorso-ventrally. That this may be considered as a remnant of 

 a once continuous fin-fold, which included both lateral and 

 median fins, has been explained above in the chapter on the 

 endoskeleton. Into this mass there grows from the ventral 

 portions of the myotomes a series of what have been aptly 

 termed myotontic buds, becoming, through bifurcation, two for 

 each somite. These continue their development until the fin 

 is supplied with a segmental series of muscle bundles, in num- 

 ber twice that of the somites from which they are derived. 

 The actual number of myotomes thus involved differs in dif- 

 ferent fishes, but in all there are certain similar principles which 

 may be brought out by the more careful study of the case 

 before us. By comparing the three diagrams in succession 

 (Fig. 57) it will be noticed that of the contribution of the first 

 trunk myotome only the second of the two buds succeeds in 

 developing, and that that one becomes abortive and fails to 

 furnish a permanent contribution to the fin. Beyond this the 

 contributions appear chronologically in regular order from the 

 front backwards, so that in A the two buds of the seventh 

 myotome and the first of the eighth have reached the fin, while 

 the second of the eighth and those of the ninth are still some 

 distance away and less completely developed. The second 

 figure, B, shows the complete disintegration of the contribu- 

 tion of the first myotome and the further development of those 

 from the second to the sixth inclusive, that is, buds II-XIII, 

 while buds XIV-XVII, or those of myotomes 8 and 9, have 

 barely reached the goal. In C is seen a single bud 



