GENERAL PRINCIPLES OF ZOOLOGY. 



either in its interior (internal plasmic products), or more often on 

 its surface (external plasmic products). The histological change is 

 the formation of specifically functioning pla&mic products. If we 

 ^ take as an example the manner in which a cell 



becomes a muscle fibre (fig. 24), we see that it 

 continually secretes upon its surface new fibrillaa 

 of specific muscle substance (in the case of the 

 vertebrates, new cross-striated muscle fibrillae), 

 until finally the remnant of the formative cell, 

 the muscle corpuscle, is contained in a mantle of 

 muscle fibrillae. In an analogous way, each 

 tissue, upon histological examination, is seen to 

 be composed of cells and plasmic products. The 

 former control the formation, the renewal, and 

 the sustenance of the tissue; the latter are the 

 agents of its physiological function. The advan- 

 tages of tissue formation are far-reaching, since 



FIG. 24. Formation & 



of muscle fibrils in m general they are connected with division of 



the frog. (Dia- & -L 



gram.) a, forma- labor (frequently referred to later). So long as 



tivecell; b, forma- .. .;, ., . ., ,,, ,, ,, ., , .. 



tive cell with two the cell unites in itself all the vital functions, 



ated S nmsciJfibrnsj these are incomplete because they mutually 



witn r ni?m V e r o^s hinder each other in their free development; the 



plasmic product, on the other hand, has only the 



single function peculiar to it and can therefore discharge its duties 



with greater completeness. The muscle fibrillae, the characteristic 



elements formed by the muscle cells, have preserved of the various 



properties of protoplasm only the capability of contraction; but 



this power of contraction is much more energetic and stronger than 



the mere movement of protoplasm. The nerve fibrillae serve only 



for the transmission of stimuli, but in an extraordinarily more 



rapid and orderly manner than does simple protoplasm. 



Classification of Tissues. Since in every tissue its function 

 interests us most, it would be natural to base the classification of 

 tissues upon the function and the intimate structure connected 

 therewith. For a long time the tissues have been arranged in four 

 groups: 1. Epithelial tissue; 2. Supporting tissue; 3. Muscular 

 tissue; 4. Nervous tissue. Within these, however, certain con- 

 stituent parts of the animal body, to which indeed the term 

 ' tissue ' is scarcely applicable, find no shelter : these are the sexual 

 cells, the blood, and the lymph. The former may be spoken of 

 in connexion with the epithelium, the latter in connexion with 

 the supporting substances. 



