I40 PAUL WEISS 



insight into the processes underlying the morphological results has 

 been obtained. There was evidence of significant changes in the distri- 

 bution of the cell content, size of the ectoplasmic border, constitution, 

 elasticity, rigidity, mobility, etc., of the cell surface, average number 

 and shape of pseudopodia, mode and rate of locomotion, viscosity, 

 vacuolization, adhesiveness, orientation of fibrillar plasma constitu- 

 ents (as indicated by silver impregnation), fat production, distribution 

 of nuclear materials, shape and hydration of nucleus and nucleoli, 

 phagocytic power, and other associated properties. In short, these cells, 

 in response to different environments, have undergone rather profound 

 reorganizations of the kind unhesitatingly classified as "differentiation" 

 or "metaplasia" by a nomenclature based on sheer appearances. 



But is this type of change directly comparable to the progressive 

 cytodifferentiation during ontogeny of which we have spoken before? 

 It evidently is not, and it is in this place that facts supplement logic. 



As we mentioned before, many of the observed changes are reversi- 

 ble if the cell is returned to its prior environment. There are many more 

 cell types of the mature organism which behave similarly in that their 

 behavior and shape change reversibly with their environment. Changes 

 in the nutrient and hormone composition of the internal milieu, sub- 

 lethal toxic agents, inflammatory processes, seasonal fluctuations, de- 

 nervation, and in fact, on a more moderate scale, practically all ordinary 

 physiological stimulations, cause a more or less thorough reshufBing 

 of the cell content with consequent change in cell behavior, and often 

 in cell form. The important point to keep in mind is that in all these 

 instances the cell can revert to its original state when the original en- 

 vironmental conditions are restored. Evidently the cell passes through 

 these transmutations with no alteration, either gain or loss, of its basic 

 equipment. At the end of a cyclic change it turns out to be the same 

 cell that it was before, and it is quite beside the point that sometimes 

 the reversal may not occur in the same cell, but in one of its descendants 

 which then prove to be "chips off the old block." There is no reason 

 why the reversible changes observed in our Schwann cells should be 

 regarded in any difTferent light than, let us say, the contractions and 

 expansions of a melanophore in response to cyclic nervous or humoral 

 stimuli, or the change in the secretory state (and associated morphol- 

 ogy) of a hormone-sensitive cell in response to varying concentrations 

 of the respective hormone. They all are merely expressions of how a 

 given cell can react to a variety of conditions — its "reaction repertory," 

 based on its material constitution such as it is at the time. The living 



