DIFFERENTIAL GROWTH I39 



tional changes in the character of cells and cell generations during their 

 life history, transformations by which they become increasingly dif- 

 ferent from their own former selves, their parent cells, and the cells of 

 other strains which have taken divergent courses. And, as we have 

 already intimated, "character" is to include the totality of reactions 

 of which the cell is capable at a given stage. In any given situation, 

 only a fraction of these possible reactions will be realized, and only a 

 small part of this fraction will become externally recognizable. 



Modulation 



We have said that cells that look alike may be intrinsically very 

 different. The opposite is equally true, namely, cells may look very dif- 

 ferent and yet be essentially alike in character. Let me cite a striking 

 example from my own experience, the vagaries of the adult sheath 

 cell of Schwann of adult mammalian nerve. In the embryonic stage 

 these cells are long spindles. They then envelop the nerve axons as thin 

 protoplasmic cylinders. When axons degenerate, as after nerve section, 

 the sheath cells regain mass, become spindly again, and glide out from 

 the nerve tubes. After nerve regeneration they resume their original 

 sheathing position and corresponding cylindrical shape. But they can 

 vary even much more than just shuttling back and forth between these 

 two states. 



Embryonic Schwann cells, reared in tissue culture along the inter- 

 face of cover glass and fluid medium, tend to transform from the 

 spindly type into a flat round form of the appearance and behavior of 

 macrophages (49). A repetition of the experiments with adult nerve 

 proved that the fully mature Schwann cell is capable of the same trans- 

 formations and more (58). Plate I illustrates a series of cell forms 

 obtained in vitro from Schwann cells of peripheral nerve. ^ They include 

 tandem strands of filamentous sheath cells, ramified astrocyte-like 

 forms, giant macrophages with rufiled membrane in active phagocyto- 

 sis, monocyte-like round cells with clear ectoplasm and eccentric nucleus, 

 and foamy round cells whose vitality is proven by their mitotic activity. 

 Each one of these phases occurs under certain definable conditions, 

 among which the fine-structural constitution of the medium seems to 

 play a predominating role. Some of the transformations are easily 

 reversible, others have as yet been observed in one direction only. Since 

 they can be followed directly as they take place in the cultures, valuable 



2 Save for a brief note, the results of these experiments have not yet been published. 



