84 



THE MECHANISM OF LIFE 



all the tissue structures which are met with in living organisms. 

 All these structures are retractile, the retraction going on 

 very slowly for a long time, as if the force of cohesion 

 continued to act in the web of the structure even after its 

 formation was complete. The phenomenon is a purely 

 physical synthetic reproduction of the phenomenon of coagula- 

 tion, the cohesion figure being in fact a retractile clot. 



Crystallization. — When we evaporate a solution oi a 

 crystalloid it becomes more concentrated, slow movements of 



Fig. 25. — The same preparation several hours later, showing a cellular 

 gastrula-like structure. 



diffusion are set up, and at a given moment agglomeration 

 occurs, the agglomerates taking the form of crystals. Thus 

 crystallization may be regarded as a particular case of con- 

 glomeration by cohesion, differing oiily in the regularity of 

 the arrangement of the molecules, which gives the geometrical 

 form of the crystal. Hence we can easily understand how 

 the presence of a crystalline fragment may facilitate the 

 process of crystallization. Consider a liquid in which 

 extremely slow movements of diffusion are taking place. 

 If the liquid is perfectly homogeneous there will be no centre 

 of attraction to which the molecules may become attached. 



