STKIATED MUSCLE TISSUE 59 



In certain heart muscle cells which are found just beneath the 

 endocardium, and are more abundant in some of the lower mam- 

 mals than in man (Purkinje's muscle fibres), the central mass of 

 undifferentiated sarcoplasm is exceptionally abundant, the fibril 

 bundles occurring only at the periphery of the cell. In many heart 

 muscle cells a small number of very fine brownish pigment granules 

 may be found in the sarcoplasm adjoining the nucleus. 



STRIATED MUSCLE TISSUE (voluntary muscle). The trans- 

 versely striated cells of this form of muscle are the most highly 

 differentiated of any of the three varieties. Their earliest an- 

 lages are found in the myotomes of the body segments, in which 

 the tissue consists of small elongated cells with a finely granular 

 cytoplasm and a single centrally situated nucleus. These cells 

 enlarge and soon present faint longitudinal striations. Coincident 

 with these changes fibril bundles make their appearance at the 

 periphery of the cell. At this stage the primitive muscle cell 

 somewhat resembles the mature type of smooth muscle. 



A 



FIG. 63. DEVELOPING MUSCLE FIBRES OF THE STRIATED VARIETY, FROM THE BUCCAL 

 MUSCLES OF A FETAL PIG. 



A, early stage, a " myoblast " ; B, later stage, fibrillation has already begun at the 

 periphery, but the central portion of the fibre is as yet undifferentiated ; the fibre has 

 been cut off at one end, the left. Hematein and congo-red. x 540. 



Further changes include the continued differentiation of the 

 cytoplasm into fibril bundles, and the rapid multiplication of the 

 cell nucleus without a corresponding division of the cytoplasm. 

 During these changes, which are accompanied by differentiation 

 of the primitive fibrils into light and dark disks, the muscle cells 

 frequently resemble the mature cardiac muscle fibres with their in- 

 distinct striations. The progress and final consummation of these 

 embryonic changes, with the consequent increase in the size of the 

 cell, produce the mature striated muscle cell with its distinct 

 membrane, its many nuclei, and its highly differentiated fibrillar 

 cytoplasm. 



The striated muscle cell is surrounded by a highly developed 

 cell wall, the sarcolemma. This structure may be seen in transec- 

 tions of the muscle fibre, but is more clearly demonstrated in 





