98 FORMATION OF CONTRACTILE TISSUE. 



rise to " epithelial cells," and that formed by those 

 beneath which are to take part in the development of 

 " fibrous tissues," " vessels," "nerves," and "muscles." 

 But gradually the soft mucus-like formed material 

 first produced, disappears, and tissue, exhibiting pe- 

 culiar structure, and manifesting special properties, is 

 slowly formed by the bioplasm. This constitutes the 

 tissue of the epithelial cell, or the fibrous tissue of 

 the subjacent textures, as the case may be. The pro- 

 cess of epithelium formation on the surface having 

 commenced, continues as long as life lasts, and the 

 loss of the old epithelial cells is compensated by the 

 production of new ones beneath. 



143. Formation of contractile tissue. One of the 

 most remarkable examples of peculiar structure 

 familiar to us, and one which cannot be at all satis- 

 factorily explained at present, is striated muscle. But 

 we must not conclude that the transverse markings 

 are essential to contractile tissue, for they are com- 

 pletely absent in the case of involuntary muscular 

 fibre. While, on the other hand, there are certain 

 kinds of fibrous tissue, destitute of contractility, 

 which possess distinct transverse markings. Nor are 

 the striae of muscle seen at an early period of develop- 

 ment. They do not make their appearance until 

 contraction of the tissue has repeatedly occurred ; but 

 the fact of their great regularity and constant uni- 

 formity in the same species of animal precludes the 

 possibility of these markings being due merely to 

 some accidental variation in the refractive power of 

 the muscular tissue. It is certain they depend upon 

 the occurrence of important structural changes 

 while the contractile material is in a very soft plastic 

 state. They may be due to the rate of formation of 

 the contractile substance, and the rapidity of the suc- 

 cessive actions of the nerve current instrumental in 

 exciting contraction. The depth of the contracting 

 portions indicated by the varying distances between 



