GENERAL PRINCIPLES OF ZOOLOGY. 



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



its surface (external j'lasmic products). Hie histological cliunge is 



the formation of specifically functioning jjlasmic jiroducts. If we 



take as an example the manner in which a cell 



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



' ■■'S continually secretes upon its surface new^ fibrillse 

 vw^S of specific muscle substance (i]i the case of the 

 sSvrt!^ vertebrates, new cross-striated muscle fibrillae), 

 until finally the remnant of the formative cell, 

 the muscle corpuscle, is contained in a mantle of 

 muscle fibrillar. In an analogous way, eacli 

 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 ° _.,... 



o( muscle fibrils in m general thev are connected with division of 



the frog. (Dia- , " , „ ", „ i x i ^ x c. i 



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



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



ateS mu'scie flbri'is- these are incomplete because they mutually 



witii°™u'ine'r o^u s hinder each other in their free development; the 



muscle flbriis. pi^gmic product, On the other hand, has only the 



single function peculiar to it and can tlierefore discharge its duties 



with greater completeness. The muscle filirilla;, the characteristic 



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



properties of protoplasm only the cai^ability of contraction; but 



this power of contraction is much more energetic and stronger than 



the mere movement of protoplasm. The nerve fibrillaj serve onlv 



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; 3. Supporting tissue; 3. Muscular 

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

 stituent parts of the animal l)ody, to which indeed the term 

 'tissue' is scarcely app]ical)lc, 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. 



