Septembek 29, 1922] 



SCIENCE 



349 



dissolved malic acid? If so, there must be a 

 man'eloiTsly arranged play of intricate se- 

 quences of eliemically attractive and repellent 

 substances dissolved suitably p»int to point 

 along the tissue. It lias recently been stated 

 tliat the nerve-fiber growing from a nerve-cell 

 in a nutrient field of graded electrical potential 

 grows strictly by the axis of the gradient. Some 

 argue for the existence of such potential gra- 

 dients in the growing organism. Certainly 

 nerve regeneration seems a return to the orig- 

 inal phase of growth, and pieces of adult tissue 

 removed from the body to artificial nutrient 

 media in the laboratory take on \agorous 

 growth. Professor Champy describes how epi- 

 ithelium that in the body is not growing, when 

 thus removed starts growing. If freed from 

 all" fibrous tissue, its cells not only germi- 

 nate, but, as they do so, lose their adult 

 specialization. In nerve regeneration the 

 nerve-sheath cells, and to some extent the 

 muscle-cells which have lost their nerve-fiber, 

 lose likewise their specialized form, and regain 

 it only after touch with the nerve-cell has been 

 re-established. So similarly epithelium and its 

 connective tissue culfevated outside the body to- 

 gether both grow and both retain their spe- 

 cialization. The evidence seems to show that 

 the mutual touch between the several cells of 

 the body is decisive of much in their individual 

 shaping and destiny. The severance of a 

 nerve-fiber is an instance of the dislocation of 

 such a touch. It recalls well-known experi- 

 ments on the segmenting egg. Destruction of 

 one of the two halves produced by the first 

 segmentation of the egg results in a whole em- 

 bryo from the remaining (half -egg; but if the 

 two blastomeres, though ligated, be left side by 

 side, each then produces a half-embryo. Each 

 half-egg can yield a whole embryo, but is re- 

 strained by the presence of the twin cell to 

 yielding but a half embryo. The nerve sever- 

 ance seems to break a mutual connection which 

 restrained cell growth and maintained cell dif- 

 ferentiation. 



It may be said that the nerve-sheath cells 

 degrade because the absence of transmission of 

 nerve impulses leaves their fiber functionless. 

 But they do not degrade in the central nerve- 

 piece, although impulses no longer pass along 



the afferent fibers. This mechanism of recon- 

 struction seems strangely detached from any 

 direct performance of function. The sprout- 

 ing nerve-fibers of a motor nerve with impulses 

 for muscular contraction can by misadventure 

 take their way to denervated skin instead of 

 muscle. They find the skin-cells the nerve<^ 

 fibers of which have been lost, and on these 

 they bud out twigs, as true sensoi-y fibers would 

 do. Then, seemingly satisfied by so doing, 

 they desist from further growth. The sense- 

 cells, too, after this misunion, regain their nor- 

 mal features. But this joining of motor nerve- 

 fiber with sense-cell is functionless, and must 

 be so because the directions of functional con- 

 duction of the two are incompatible. Similarly 

 a regenerating skin-nerve led down to muscle 

 makes its union with muscle instead of skin, 

 though the union is a functional misfit and 

 can not subserve function. Marvelous though 

 nerve regeneration be its mechanism seems 

 blind. Its vehemence is just as great after 

 amputation, when the parts lost can of course 

 never be re-reached. Its blindness is sadly 

 evident in the suffering caused by the useless 

 nerve-sprouts entangled in the scar of a heal- 

 ing or healed limb-stump. 



There is a great difference, however, between 

 the growth of such regeneration and the 

 growth impulse in x^ieees of tissue isolated from 

 the body and grown in media outside. With 

 pure cultures, in the latter case, Professor 

 Champy says the growth recalls in several 

 features that of malignant tumors, for exam- 

 ple, multiplication of cells unaccompanied by 

 formation of a specialized adult tissue. A piece 

 of kidney cultivated outside the body de- 

 differentiates, to use his term, into a gi'owing 

 mass unorganized for renal function. But 

 with connective-tissue cells added even breast- 

 cancer epithelium will in cultivation grow in 

 glandular form. New ground is being broken 

 in the experimental control of tissue growth. 

 The report of the Imperial Cancer Eeseareh 

 Fund mentions that in cultivation outside the 

 body malignant cells present a diffculty that 

 normal cells do not. To the malignant cells the 

 nutrient soil has to be renewed more frequently, 

 because they seem rapidly to make the soil in 

 which they grow poisonous to themselves. 



