668 THE MUSCULAR FIBRES. 



muscular system of other types is still very obscure. The 

 muscles may arise in the embryo from amoeboid or indifferent 

 cells, and the Hertwigs 1 hold that in many of these instances the 

 muscles have also phylogenetically taken their origin from 

 indifferent connective-tissue cells. The subject is however beset 

 with very serious difficulties, and to discuss it here would carry 

 me too far into the region of pure histology. 



The voluntary muscular system of the CJiordata. 



The muscular fibres. The muscular elements of the 

 Chordata undoubtedly belong to the myo-epithelial type. The 

 embryonic muscle-cells are at first simple epithelial cells, but 



FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after 

 O. and R. Hertwig.) 



A. Muscle-cell from the circular fibres of the subumbrella. 



B. Myo-epithelial cells from the base of a tentacle. 



soon become spindle-shaped : part of their protoplasm becomes 

 differentiated into longitudinally placed striated muscular fibrils, 

 while part, enclosing the nucleus, remains indifferent, and con- 

 stitutes the epithelial element of the cells. The muscular 

 fibrils are either placed at one side of the epithelial part of the 

 cell, or in other instances (the Lamprey, the Newt, the Sturgeon, 

 the Rabbit) surround it. The latter arrangement is shewn for 

 the Sturgeon in fig. 57. 



The number of the fibrils of each cell gradually increases, 

 and the protoplasm diminishes, so that eventually only the 

 nucleus, or nuclei resulting from its division, are left. The 

 products of each cell probably give rise, in conjunction with a 

 further division of the nucleus, to a primitive bundle, which, 



1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881. 



