68 



NORMAL HISTOLOGY. 



FIG. 81. 



within certain areas, of the irregular mesodermic elements into the 

 elongated fusiform fibre-cells. In suitable preparations all gradations 

 between the ordinary embryonal connective-tissue cells and the 

 muscular elements may be observed, emphasizing the common 

 ancestry of the two forms of tissue. 



Voluntary muscle, representing a higher specialization, is de- 

 rived from definite areas constituting the inner layer of the muscle- 

 plates, which are referable to the 

 early stages of the primary segmenta- 

 tion into somites. The cells of the 

 muscle-plate soon elongate, with pro- 

 liferation of the nuclei, to become the 

 primitive muscle-fibres. These at 

 first consist of greatly extended ele- 

 ments, possessed of numerous nuclei 

 and composed of granular indifferent 

 protoplasm. After a time the fibre 

 exhibits a differentiation into longi- 

 tudinal striae, which, later, are supple- 

 mented by the transverse markings 

 characteristic of voluntary muscle. The 

 sarcolemma appears about the time 

 the longitudinal markings are seen. 



The striations are limited, at first, 

 to one side of the fibre, then extend 

 over the entire periphery, but still for 

 some time do not reach the centre of 

 the fibre, an inner zone of undiffer- 

 entiated sarcoplasm occupying the 

 middle. Later, this area also becomes 

 converted into striated tissue, while the 

 once numerous nuclei are reduced to the 

 few collected beneath the sarcolemma. 

 Cardiac muscle, likewise, develops 



from the mesoderm immediately surrounding the primary heart-tubes, 

 the contractions of the cells being displayed even before the histo- 

 logical differentiation becomes apparent. In its development it 

 represents an intermediate stage, since the original spindle-cells be- 

 come converted into protoplasmic fibres containing a central area 

 which always remains less differentiated and nearer its primary con- 

 dition of indifferent sarcoplasm than the peripheral portions of the 

 fibre. The fibres of Purkinje, found in the hearts of certain rumi- 

 nants, represent muscular fibres in which the sarcoplasm remains in 

 part still undifferentiated. 



Developing voluntary muscle: A, 

 young muscle-cells ; a, very young 

 spindle-cell ; b, older element, exhibiting 

 indications of future striation on one 

 side ; the remaining part of the cell is 

 composed of the undifferentiated sarco- 

 plasm ; B, embryonal muscle-fibres pos- 

 sessing many nuclei and traces of striae ; 

 C, developing muscle-fibres in section ; 

 in the larger fibres a differentiated 

 peripheral zone of striae (d) is seen in 

 section ; an area of still indifferent sar- 

 coplasm occupies the centre of the fibre 

 and surrounds the nucleus (). 



