22 SENESCENCE AND REJUVENESCENCE 



for the phenomena of life. The organic colloids are chiefly proteid 

 or fatty in nature, and the present state of our knowledge indicates 

 that the properties of these substances as colloids are no less im- 

 portant for the living organism than their chemical constitution. 



In every living organism known to us the chemical processes 

 of metabolism take place in a complex colloid field or substratum, 

 and many of the peculiarities of the metabolic processes are unques- 

 tionably due to this fact. Within recent years the significance of 

 colloids for the phenomena of life has been pointed out again and 

 again. Bechhold in his recent book ('12) goes so far as to assert 

 that life is inconceivable except in a colloid system. Doubtless 

 "colloid chemistry" is at present the fashion, but it is also true that 

 this fashion has a certain justification. The study of the behavior 

 and properties of colloids has thrown new light, not only on many 

 problems of chemistry and physics, but on many problems of biology 

 as well. Attention may briefly be called to a few of these biological 

 problems. 



The problems of localization and morphogenesis assume a new 

 form in the light of our knowledge of colloids. In the course of 

 development of the organism certain processes become localized 

 at certain points and morphological structure and differentiation 

 result. The visible basis of morphogenesis is the protoplasm, and 

 in it the structural features arise. The definiteness and persistence 

 of organic structure in a substance like protoplasm which presents 

 all conditions between a concentrated and a very dilute gel or a sol 

 has always presented many difficulties, and the problem is at 

 present by no means solved. The attempt has been made repeat- 

 edly to find in the process of crystallization and the definiteness of 

 form in the crystal a basis for organic form and structure, but with- 

 out any very satisfactory results. The resemblance between the 

 physical process of crystallization in a substance of uniform consti- 

 tution and the development of form and structure in connection 

 with chemical reaction in the complex organism is certainly not 

 very close. 



Under proper conditions it is possible to produce more or less 

 definite forms by means of chemical reaction, but in all such cases 

 we find that the form is not directly dependent upon the reaction 



