THE PRESIDENTIAL ADDRESS. 7 



tion of such atoucli. It recalls well-known experiments on the segment- 

 ing egg. Destruction of one of the two halves produced by the first 

 segmentation of the egg results in a whole embryo 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 restrained by the presence of the twin cell to yielding but 

 a half one. The nerve severance seems to break a mutual connection 

 which restrained cell growth and maintained cell differentiation. 



It may be said that the nerve-sheath cells degrade because absence 

 of transmission of nerve impulses leaves their fibre functionless. But 

 they do not degi^ade in the central nerve-piece, although impulses no 

 longer pass along its afferent fibres. This mechanism of reconstruction 

 seems strangely detached from any direct performance of function. The 

 sprouting nerve-fibres of a motor nerve with impulses for muscular con- 

 traction can by misadventure take their way to denervated skin instead 

 of muscle. They find the skin-cells whose nerve-fibres have been lost, 

 and on these they bud out twigs, as true sensory fibres would do. Then, 

 seemingly satisfied by so doing, they desist from further growth. The 

 sense-cells, too, after this misunion, regain their normal features. But 

 this joining of motor nerve-fibre with sense-cell is functionless, and 

 must be so because the directions of functional conduction of the two 

 are incompatible. 



So, 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 cannot subserve function. Marvellous though nerve re- 

 generation 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 healing or healed limb- 

 stump. 



But there is a great difference between the growth of such regenera- 

 tion and the growth impulse in pieces of tissue isolated from the body 

 and grown in media outside. With pure cultures of these latter 

 Professor Champy says the growth recalls in several features that of 

 malignant tumours — multiplication of cells unaccompanied by forma- 

 tion of a specialised adult tissue. A piece of kidney cultivated outside 

 the body de-differentiates, to use his temi, into a growing mass un- 

 organised 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 Research Fund 

 mentions that in cultivation outside the body malignant cells present 

 a difficulty that normal cells do not. To the malignant cells the nutrient 

 soil has to be more frequently renewed, because they seem rapidly to 



