THE BIOLOGY OF THE CELL SURFACE 



beyond it, we must realize this fact. For this reason, the 

 most definite knowledge of the manifestations of the normal 

 living state becomes necessary. Experiments in biology, 

 then, fall into three categories: 



1. Experiments on non-living systems. 



2. Experiments on killed living systems. 



3. Experiments on living systems. 



Number i includes experiments that furnish comparisons 

 between living and non-living systems and that elucidate 

 the result of cellular activity which expresses itself in secre- 

 tion and excretion. Here belong physical models and 

 artificial systems, as for example, semi-permeable mem- 

 branes, suspensions of soap in water — used to imitate 

 cleavage-patterns of the egg — colloidal solutions, passive 

 iron wire — employed to illustrate nerve-conduction — etc. 

 Also, here belong chemical analyses of substances that are 

 produced by the living cell, as for example, hormones and 

 digestive ferments and excretions, as urine, etc. 



Number 2 includes experiments on the chemical composi- 

 tion of living matter, killed in the process of analysis. 

 Though not experimental in the strict sense of the word, all 

 histological and cytological studies on fixed tissues and cells 

 are included in this category. When properly used, fixa- 

 tion gives a faithful picture of the living and as such has 

 great value for the study of it. 



Number 3 embraces experiments on the living system. 

 These are of two kinds: those in which the normality 

 remains unchanged and those in which it is altered. By 

 means of the former, valuable data are collected by which 

 indicia and criteria are established for supplementing and 

 extending as well as interpreting observations and descrip- 

 tions of vital processes such as respiration, etc. Such 

 experiments are also of incalculable value for medicine. 

 By the second type of experiments in this category we 

 often succeed in making evident the roles of various factors 



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