60 LIFE: ITS NATURE AND ORIGIN 



tions that result go to make up the various characteristic struc- 

 tures we recognize at higher structural levels, many of which have 

 long been the wonder of microscopists because of the meticulous 

 accuracy of their formation. The electron microscope reveals 

 some of these "vestiges of molecular creation" at submicroscopic 

 levels, as may be seen in Fig. 4, micrographs of collagen. It 

 reveals that this supposedly "homogeneous" substance, in the 

 specimens examined, consists of two substances in regular forma- 

 tion, one of which is more readily stretched than the other. This 

 causes the striations so apparent in the picture. The biocatalysts 

 determine where and what substances shall be formed, and the 

 formation rates; and the subsequent mutual reactions of the sub- 

 stances largely determine what superior structures will emerge. 



Some insight as to the further complications that arise at a still 

 higher structural level when numerous cells get together is given 

 by the slime molds. When plenty of food is available, they live 

 singly and divide by fission. After this vegetative stage, the cells 

 gather together in masses, which may number as high as 150,000 

 cells, and form what is termed a pseudoplasmodium, which moves 

 about as though it were an individual unit. In the case of 

 Dictyostelium discoideum 42 the migrating unit stops just before 

 the formation of a spore-bearing stalk (sorocarp). After aggrega- 

 tion of the individual cells (myxameboe) there is no further 

 increase in their number or size; but all further development 

 consists entirely in the integration and subsequent differentiation 

 of the myxameboe already present, which may be of wholly differ- 

 ent spore origin and of initially equal potentialities. If the cell 

 masses are broken up in the presence of food (bacteria), they re- 

 turn to the vegetative self-reproducing stage; but if no bacterial 

 food is present, they re-aggregate and develop fruiting structures 

 whose pattern is constant, though the size is in proportion to the 

 cell mass. Different species, thoroughly intermixed in the vegeta- 

 tive stage, aggregate to separate and distinct centers. Though 

 two species of Dictyostelium may initially enter the same fruiting 

 organization, they later draw apart and form separate sorocarps, 

 which have a distinctive form for each species. "What is inherited 

 is not a specific type of structure, but the ability of like but discrete 

 and independent units to cooperate in the formation of such a 

 structure." 



Just as slowly grown crystals tend to "purify" themselves by 

 elimination of stranger ions or molecules from their space lattices, 



