THE CELL AND PROTOPLASM. 



87 



fied to perform different functions. The different 

 organs appear within the cell, and the cell is 

 more complex than the tj 7 pical cell described. 

 Fig. 21 shows such a cell. Such an animal 

 possesses several or- 

 gans, but, since it 

 consists of a single 

 mass of protoplasm 

 and a single nucleus, 

 it is still only a 

 single cell. In the 

 multicellular organ- 

 isms the organs of 

 the body are made 

 up of cells, and the 

 different organs are 

 produced by a dif- 

 ferentiation of cells, 



but in the Unicel- FIG. 21.-A complex cell. It is an 



lular organisms the onlyVe^eii!' but composed of 

 organs are the re- 

 sults of the differentiation of the parts of a 

 single cell. In the one case there is a differentia- 

 tion of cells, and in the other of the parts of a 

 cell. 



Such, in brief, is the cell to whose activities 

 it is possible to trace the fundamental properties 

 of all living things. Cells are endowed wiih the 

 properties of irritability, contractibility, assimila- 

 tion and reproduction, and it is thus plainly to 

 the study of cells that we must look for an in- 

 terpretation of life phenomena. If we can reach 

 an intelligible understanding of the activities of 

 the cell our problem is solved, for the activities 



