CELLULAR DIFFERENTIATION. 53 



as a specialization, and limitation in direction, of the power of 

 contraction which we have seen to be resident in all living 

 protoplasm. Muscular tissue differs somewhat in structure 

 and degree of differentiation in various animals, but in general 

 agrees in the presence of elongated fibres which are to be 

 considered as modified cells or parts of cells. The contractile 

 muscle substance is, in part at least, a plasmic product rather 

 than mere protoplasm; yet it differs from the plasmic inter- 

 cellular substance of the tissues already described in that it is 

 deposited within rather than among the cells. Two stages 

 in the differentiation of muscular substance are to be noted : 

 ( i ) the fibres may be plain, in which case we find elongated, 

 contractile single cells without conspicuous external differen- 

 tiation (Fig. 27) ; (2) cross-striate fibres, which always show 

 conspicuous differentiation of parts in each fibre as seen under 

 the microscope. The plain fibres are characteristic of slug- 

 gish animals, and those parts of animals whose muscular 

 action is least prompt in response to the nervous stimuli (e. g., 

 digestive tract in vertebrates). The cross-striated fibre usu- 

 ally represents several incompletely separated cells, or a multi- 

 nucleate condition of a much-grown and metamorphosed single 

 cell. In both classes the fibres are made up of numerous 

 minute strands or fibrilla? which in the plain muscle are homo- 

 geneous throughout, but in the cross-striated are made up of 

 alternating segments of lighter and darker optical appear- 

 ance (Fig. 28, B}. The undifferentiated protoplasmic rem- 

 nant is often very small in amount, and is collected about the 

 nucleus (Figs. 27, 28). It may be at the surface of the fibre 

 or in the centre, enveloped by the contractile matter. A thin 

 membrane (sarcolemma) binds the fibrillse into fibres. The 

 fibres are bound together by strands of connective tissue into 

 bundles, and of these bundles the muscle is made up. 



79. Origin of Muscle Tissue. In those animals in which a true meso- 

 derm is wanting, the epithelial cells may develop, at their inner extremity, 

 contractile roots, either plain or striate (Hydra, Fig. 17, B). These cells 

 may wholly lose their epithelial quality and position and become entirely 

 muscular. In the higher animals this is very much modified by the early 



