MUSCLE TISSUE 



127 



changes in shape, delicate longitudinal fibrils appear in the cytoplasm. Just 

 how these fibrils originate is not known, but they probably represent a speciali- 

 zation and rearrangement of the spongioplasm. 



t:.:rr>i^> 



ii?S 



Fig. 71. — Myoblasts in Different Stages of Development. (Godlewski.) 

 The upper cell represents a myoblast with granular cytoplasm (from sheep embryo 

 of 13 mm.); the middle, a myoblast with fibrils in process of formation (from guinea-pig 

 embryo of 10 mm.); the lower, a myoblast with still further differentiated, segmented 

 fibrils (from a rabbit embryo of 8.5 mm-). 



Striated Voluntary Muscle. — This develops from the mesoblastic somites. 

 Each somite early divides into an outer part, sclerotome or cutis plate, and an inner 

 part, the myotome or muscle plate. Cells of the myotome soon show changes 





Fig. 72. — From a Section of Developing Heart Muscle from a Rabbit Embryo of 9 



mm. (Godlewski.) 



a, Cell body with granules arranged in series; b, cell body with centrosome and attrac- 

 tion sphere; c, branching fibril; d, fibrils extending through several cells. 



which distinguish them as myoblasts or muscle-forming cells. These cells, which 

 are at first spherical, become elongated and spindle shaped. The nucleus is at 

 this stage centrally placed and the spongioplasm is in the form of a reticulum. 



