DIFFERENTIAL GROWTH I45 



somatic differentiation. Accordingly each nucleus contains both "dif- 

 ferentiated" and "undifferentiated" portions. 



The question arises whether the cytoplasm might not be similarly 

 subdivided into a differentiated fraction and an undifferentiated residue. 

 Such is generally conceded to be the case for the cytoplasm in the "germ 

 track," i.e. of those cells that lie in direct ascendancy of the primordial 

 germ cells. The same must be assumed for those "totipotent" somatic 

 cells of the lower invertebrates which can give rise to a new individual 

 by budding, regeneration, or other forms of asexual reproduction. The 

 specialized somatic cells of higher forms, on the other hand, have never 

 given any evidence of such totipotence. This could be interpreted to 

 indicate that all of their cytoplasm has been diverted into a specialized 

 course, has become liver-specific for the liver cell, kidney-specific for 

 the kidney cell, and so forth. Yet again, there is undeniably some stock 

 'of cytoplasmic equipment that practically all cells have in common (e.g. 

 respiratory enzymes, mitochondria, centrosomes, spindle fibers, etc.) and 

 which, thus, seems to have escaped the progressive changes of cytodif- 

 ferentiation. 



In short, any attempts at making general statements for the nucleus 

 as a whole or the cytoplasm as a whole end up in contradictions. The 

 more concretely and specifically we try to state our problems, the more 

 clearly we realize the inadequacy and ambiguity of our traditional 

 vocabulary. So long as we address our questions not to a real living cell, 

 but to a vague and abstract symbol, we can expect no more than oracular 

 replies. Those general concepts of cell and cytoplasm with which we 

 conventionally operate, with their sundry textbook attributes of con- 

 tractility, excitability, adaptability, and so forth, have just about lived 

 out the term of their usefulness. We no longer view contractility and 

 excitability vaguely as "fundamental attributes of protoplasm," but 

 have recognized them as defined properties of circumscribed physico- 

 chemical systems, protein chains and surface membranes, respectively. 

 Differentiation deserves a similarly specific treatment. This will be 

 feasible only if we replace the abstract notion of the cell by a much more 

 specific picture. Such a picture is gradually emerging from the combined 

 efforts of cytology, general physiology, and biochemistry. According to 

 this picture, a cell is made up of known or knowable molecular elements, 

 and cellular activities are resolvable into elementary molecular processes. 



Study of these elementary processes is well under way and has yielded 

 spectacular results. However, preoccupation with the elements has also 

 at times tended to obscure the fact that in order to constitute a living 



