l66 PAUL WEISS 



ences in the efficiency of utilization of foodstuffs, and by differential 

 growth stimulators and growth inhibitors.^ Plainly, many of these 

 theories miss the point in that they try to explain differences which only 

 appear to be differences of basic growth but in reality are natural results 

 of differentiation. Further compelling evidence on this point will be 

 presented below, where we shall deal with tissue growth. 



The Site of Growth 



The distinction between a growing {R-) and non-growing {D-) frac- 

 tion in protoplasm raises the question of the sites of growth. Evidently, 

 the R- and D-fractions might be either segregated or interspersed. They 

 might be mingled as mosaics of molecular, submicroscopic, or micro- 

 scopic (particulate) dimensions; they might be uniformly dispersed or 

 each concentrated in certain localities, which might or might not corre- 

 spond to the nucleus, the chromosomes, nucleolus, cell surface, or other 

 topographical features. 



Some unexpected light has been shed on these problems from recent 

 observations on nerve growth, which I shall briefly relate. A nerve fiber 

 is the long cytoplasmic extension of a nucleated cell body, surrounded 

 by sheath cells. The fiber grows enormously in length and width during 

 ontogeny, and even more dramatically during regeneration. When a fiber 

 is cut, the distal stump, severed from its nucleated base, perishes, while 

 the proximal stump, still connected with its central cell, grows out anew. 

 The rapid elongation of the regenerating fiber is a process of proto- 

 plasmic movement rather than of growth in the proper sense (51 )• But 

 true growth also supervenes and becomes most conspicuous in the in- 

 crease in width of the fiber (from less than i micron to often more 

 than 10 micra, i.e. more than a hundred times in mass). Since the fiber 

 is surrounded by accessory sheath cells and well supplied with blood, one 

 could well imagine that its growth is sustained from local sources. The 

 experimental evidence, however, is that growth takes place exclusively 

 in the nucleated central part of the cell and that all the specific cytoplasm 

 for the whole axon is synthesized there. The evidence comes from con- 

 striction experiments as follows (55). 



A local constriction (by a ring of artery) reduces the diameters of 

 the neurilemmal tubes which contain the nerve fiber. Fibers which have 

 regenerated through such "bottlenecks" remain permanently undersized 

 from that point on distally. In contrast, these same fibers swell to ex- 

 cessive dimensions just proximal to the constriction. Plate III shows 



8 For some recent examples, consult Needham (29) and Spiegelmann (38). 



