CELLULAR ORGANIZATION OF LIVING MATTER 15 



able, however, if micrococci, and especially the organisms 

 in filterable viruses, can be considered as cells in the 

 true sense. The case of the ultra-microscopic organisms 

 present in the filterable viruses is of special interest. 

 These organisms can be demonstrated only by the effects 

 which they produce (infection); they prove themselves 

 to be living by their power of automatic growth, shown 

 by multiplication in the body of the host or in culture- 

 media, and also by exhibiting other properties character- 

 istic of protoplasm in general, such as thermolability 

 and susceptibility to toxic agents of the disinfectant 

 class. They may be described as complex and chemically 

 active (metabolizing) material in a fine state of sub- 

 division (like that of colloidal material), possessing in 

 addition to the other properties of matter in this state 

 the special vital properties of assimilation, growth, and 

 multiplication. As already pointed out, this ability to 

 transform environmental material into its own specifically 

 organized and active substance is the distinctive criterion 

 of living as distinguished from non-living matter. 



Our conceptions of the nature of living organisms 

 must be broad enough to include the ultra-microscopic 

 forms. Cells, as found in higher organisms, are units 

 of a relatively complex and highly differentiated kind, 

 representing a comparatively advanced stage of evolu- 

 tion. They are by no means to be regarded as the only 

 systems in nature exhibiting the characteristics of life. 



THE CELL 



In higher organisms, however, we have definite 

 experimental evidence that the smallest unit capable of 

 continued independent life is the nucleated cell. The 



