NATURE OF LIFE AND LIVING MATERIAL 19 



Physical Structure of Protoplasm. In appearance proto- 

 plasm is unimpressive (Fig. i). Under the microscope there is 

 nothing particularly striking in its translucent, jelly-like appearance. 

 But its peculiar powers always prompt a searching analysis of its 

 make-up. The question is always uppermost: What structure or 

 arrangement in this watery, gelatinous mass is the seat of its life 

 processes? It was not until the latter part of the nineteenth century 

 that the physical nature of protoplasm was 

 determined; its most intimate details still 

 elude analysis. Prior to this time observa- 

 tions and speculations had led to various 

 conclusions concerning its structure. By 

 some it was thought to be a mass of liv- 

 ing fibres in a watery liquid; others ob- 

 served granules and concluded that the 

 living portion was granular, the granules 

 being suspended in a watery liquid. Other 

 observations showed that the structure is 

 that of a foam, the minute cells of the 

 foam filled with fluid. But in the last years 

 of the century observations on both living 

 and carefully killed and prepared proto- 

 plasm showed that all these pictures could 

 be found in the same protoplasm at different times. It is now gen- 

 erally agreed that the whole of protoplasm is essential to its life 

 and that it consists of visible fibres, foams, and granules, in a watery 

 medium, the exact structure of which is not known but which is 

 undoubtedly a complex colloid. 



Colloids. Colloids were first defined by a chemist in 1861. 

 Since the living substance of all living objects, • Man as well as 

 the most obscure plant, is colloidal, some understanding of the 

 nature and characteristics of colloids is essential to an appreciation 

 of the principles of Biology. A colloid in the broadest sense consists 



Fig. I, — Living human 

 protoplasm as it appears 

 when examined under a 

 high power of the micro- 

 scope. The specimen was 

 taken from the lining of 

 the mouth and put under 

 light pressure; the field of 

 observation was intensely 

 illuminated. 



