6 



HUMAN BIOLOGY 



know, that the most important part of the cell is not the 

 lifeless wall of cellulose, but the living substance which is 

 found inside the cell wall, making up a large part of the cell 

 body and cell nucleus. To this substance is given the name 

 protoplasm. We know now that the living substance or pro- 

 toplasm is the essential part, while the wall may be missing, 

 so that in such a case there is no resemblance to a cell or 

 box. Biologists now understand a cell to be a bit of proto- 

 plasm (cell body) containing a nucleus (which is a denser por- 

 tion of the protoplasm). 



Protoplasm, when examined with the highest powers of 

 the microscope, appears as a colorless, semifluid substance, 

 in which are often seen solid particles or granules, which are 

 probably little masses of food. The nucleus, as already 

 stated, is commonly found near the center of the cell, and is 

 composed of protoplasm denser than the protoplasm of the 

 rest of the body of the cell. The appearance and composition 

 of the protoplasm surrounding the nucleus, that is, the cell 

 body, may be well represented by raw white of egg ; but in 

 making this comparison one should bear in mind that the 

 white of an egg is not living substance. 



6. Assimilation, growth, and cell division. Within the pro- 

 toplasm are foods in solution (such as sugar, protein, and 

 mineral matters). These are used by cells in their growth 

 and repair, and in the various kinds of work that they carry 

 on. In the human body, as in plants, the food materials 

 are gradually changed by protoplasm into living substance 

 like itself. To this process is given the name assimilation 

 (Latin, ad = to + similis = like). As a result of the process 

 of assimilation the amount of protoplasm of course increases 

 and the cell grows. Were this process to continue indefi- 

 nitely, cells would come to be large in size. This, however, 



