IV 



MYOTOMES 



203 



Some such mesenchyme cells also penetrate into the substance of the 

 myotome and settle down there to form connective tissue. The 

 cylindrical myoblasts of the outer wall undergo active multiplication 

 (Fig. 110, C) so that it comes to be greatly thickened, composed of 

 many layers of muscle-cylinders — those towards the outer surface 

 going on dividing actively while those further in towards the mesial 



vac. 



mb 



mb 



mf 



mb 



mb 



Pig. 111. — Differentiation of the myotome as seen in horizontal sections of 

 Lepidosiren larvae. 



A, stage 31 ; B, stage 31.+. mV, myoblast of inner wall ; mb", myoblasts of outer wall ; 

 mf, contractile fibrils ; vac, vacuoles ; y, yolk. 



plane increase much in size as they develop more and more fibrils in 

 their interior. 



As the outer wall of the myotome continues to increase in 

 thickness the myoblasts of the inner wall become relatively more 

 and more insignificant. Eventually they divide up into muscle- 

 cylinders like those of the outer wall so that it is no longer possible 

 to distinguish the inner wall portion of the myotome from the outer 

 wall part. The muscle-cylinders become the muscle-fibres of the 

 adult, the undifferentiated protoplasm between the fibrils persisting 

 as the sarcoplasm the superficial layer of which may be somewhat 

 condensed to form the sarcolemma. 



