CELLS AND THE CELL THEORY 49 



The explanation of its activities can no longer be regarded as a 

 chemical problem simply, but must be in a measure a mechanical 

 problem as well. This conception totally alters the significance 

 of the phrase "the physical basis of life" and puts the prob- 

 lem of the mechanical theory upon a decidedly new footing. 



To-day biologists are gradually giving up the use of the term 

 protoplasm as confusing and misleading, replacing it by more 

 definite terms which refer directly to the different parts of the 

 cell. So now we find coming into general use the terms cytoplasm 

 and karyoplasm (see page 32) to cover what was formerly called 

 protoplasm. Both cytoplasm and karyoplasm are necessary 

 and must act together in order to show the general characters 

 of life. That reproduction may occur, the chromatin, and per- 

 haps the centrosome also, are requisite. 



The mechanical theory is no longer tenable in the form in 

 which it was originally advocated and discussed. That position 

 has been necessarily abandoned since the studies of more recent 

 years have demonstrated that protoplasm is not a homogeneous 

 substance and cannot be regarded simply as a chemical com- 

 pound. It is", on the contrary, a very complex mixture of sub- 

 stances, forming a complicated machine in which the parts are 

 most intricately interrelated and adjusted. While chemical forces 

 may be regarded as sufficient to manufacture almost anything 

 in the way of chemical compounds, they are not adapted to the 

 manufacture of such a mechanism as living protoplasm has 

 been proved to be. This change in the attitude of biologists 

 has been brought about mainly through the minute study of 

 the nucleus and the constantly increasing recognition of its 

 great importance in the life of the cell. 



Are There Life Units Simpler Than Cells? As we have 

 learned, the cell is by no means a simple structure but a compli- 

 cated mechanism. The question inevitably arises whether the 

 cell is the simplest structure that can manifest life or whether it 

 may not be analyzed into simpler units. This is one of the 

 puzzling and unsettled problems of biology. Certainly some of 



