60 



GENERAL PHYSIOLOGY 



shows that it is composed of various constituents, which are termed 

 tissues. The arm contains muscle-tissue, nerve-tissue, bone-tissue, 

 etc. ; the characteristic of the organ is its composition out ot 

 one' or more tissues. The next lower stage of indi- 

 viduality, therefore, is the tissue. Certain organisms consist of 

 but a single tissue, in which all the constituents are alike. Such 

 free-living tissues are widely represented among the Algce. 

 Eudorina elegans, e.g., is a small transparent ball of jelly, in which 

 many spherical particles lie embedded, which upon close examina- 

 tion prove to be bits of living substance separated from one 

 another. These single minute particles of living substance are 

 termed cells. In this particular case each cell has two delicate 

 flagella, by the movement of which the whole mulberry-mass of 



jelly is driven about in 

 the water (Fig. 5). Every 

 such flagellate cell is an 

 independent individual, 

 and continues to live 

 when separated from the 

 ball of jelly, which hap- 

 pens, e.g., spontaneousl}- 

 in reproduction. It is 

 seen, therefore, that the 

 tissue contains within it- 

 self the single cell. The 

 tissue is a colony- of cells. 

 In the cell the lowest 

 stage of individuality has 

 been reached. The cell 

 is, indeed, composed of 

 various constituents, of a 

 soft ground - substance, 

 the protoplasm, and a 

 . more solid cell-nucleus 



embedded m it; but m no case can these two constituents be 

 separated without the death of both. Many experiments have 

 shown that protoplasm is incapable of self-preservation without the 

 cell-nucleus, and the nucleus similarly incapable without the proto- 

 plasm. Hence, according to the above definition of individuality 

 neither of the two represents an individual. In all nature lio 

 organism is known which represents a lower stage of individualitA" 

 than the cell. As Briicke ('61) says, the cell is the "elementary 

 organism." "^ 



Apparently in contradiction with this idea is the fact, recently 

 established by many experiments, that under certain conditions 

 the cell can be artificially divided into pieces which continue to 

 live and even reproduce. If, e.g., a free-living infusorian cell, such 



Fio. 5.' — Eudorina elegans, a colony of Flagcllata. 

 The single individuals lie embedded in a common 

 ball of jelly. 



