230 DEVELOPMENT OF MUSCULAR TISSUE. 



characters of the embryonic fibres of the higher verte- 

 brata. Some of the muscular fibres of the adult 

 frog and hyla have masses of bioplasm in the centre 

 of the elementary fibre, as just described ; so also 

 have the muscular fibres of the heart of the human 

 subject. The fibre increases in diameter by the forma- 

 tion of new contractile tissue within, which is formed 

 upon the surface of the bioplasm, and the contractile 

 tissue which had been produced previously is pushed 

 outwards. Many muscular fibres as, for instance, 

 those of the delicate muscles of the eye of the smallest 

 animals exist at an early period as spindle-shaped 

 bodies, which taper at either extremity into the 

 tendon. The large mass of bioplasm is in the centre, 

 and is surrounded by formed material, which gra- 

 dually accumulates upon its surface and at its two 

 extremities. Thus the fibre increases in thickness 

 and length. 



In the connective tissue of the nose of the nearly 

 full-grown mole (prep. 103) the development of mus- 

 cular fibre may be well studied, for in this situation 

 are numerous bundles of very narrow, but distinctly 

 transversely striated, muscular fibres, which taper at 

 either extremity into tendons of great length which 

 pass into the connective tissue. 



In most of the permanent elementary muscular 

 fibres of the higher vertebrate animals the masses of 

 bioplasm are seen at intervals embedded in the con- 

 tractile tissue, and disposed in much the same manner 

 as the masses of bioplasm of tendon. In exceedingly 

 fine fibres, an oval mass of bioplasm is often present 

 upon one side only. In fibres of about the g^V^-th 

 of an inch in diameter, and apparently composed of 

 only a very few fibrilla?, I have seen the oval mass of 

 bioplasm situated a short distance from the side of 

 the contractile tissue, with which it was connected 

 by a small quantity of exceedingly delicate granular 

 matter, exhibiting here and there indications of 



