i88 



NATURE 



[October 14, 191, 



with fixity of substance; but the material with which 

 the biologist is occupied consists of innumerable living 

 unit-individuals exhibiting specific characteristics 

 without fixity of substance. There is no reason 

 to suppose that the properties of a given chemical 

 substance vary in the slightest degree in space or 

 time; but variability and adaptability are character- 

 istic features of all living beings. The biochemist 

 renders inestimable services in elucidating the 

 physico-chemical mechanisms of living organisms; 

 but the problem of individuality and specific be- 

 haviour, as manifested by living things, is beyond 

 the scope of his science, at least at present. Such 

 problems are essentially of distinctively vital nature, 

 and their treatment cannot be brought satisfactorily 

 into relation at the present time with the physico- 

 chemical interactions of the substances composing the 

 living body. It may be that this is but a temporary 

 limitation of human knowledge prevailing in a certain 

 historical epoch, and that in the future the chemist 

 will be able to correlate the individuality of living 

 beings with their chemico-physical properties, and so 

 explain to us how living beings first came into exist- 

 ence ; how, that is to say, a combination of chemical 

 substances, each owing its characteristic properties 

 to a definite molecular composition, can produce a 

 living individual in which specific peculiarities are 

 associated with matter in a state of flux. But it is 

 altogether outside the scope and aim of this address 

 to discuss whether the boundary between biochemistry 

 and biology can be bridged over, and if so, in what 

 way. I merely wish to emphasise strongly that if 

 a biologist wishes to deal with a purely biological 

 problem, such as evolution or heredity, for example, 

 in a concrete and objective manner, he must do so 

 in terms of living specific individual units. It is for 

 that reason that I shall speak, not of the chromatin- 

 substance, but of chromatinic elements, particles or 

 units, and I hope that I shall make clear the import- 

 ance of this distinction. 



To return now to our chromatin ; I regard the 

 chromatinic elements as being those constituents 

 which are of primary importance in the life and 

 evolution of living organisms mainly for the follow- 

 ing reasons : the experimental evidence of the pre- 

 ponderating physiological r6\e played by the nucleus 

 in the life of the cell; the extraordinary individual- 

 isation of the chromatin-particles seen universally in 

 living organisms, and manifested to a degree which 

 raises the chromatinic units to the rank of living in- 

 dividuals exhibiting specific behaviour, rather than 

 that of mere substances responsible for certain 

 chemico-physical reactions in the life of the organism ; 

 and last, but by no means least, the permanence and, 

 if I may use the term, the immortality of the chro- 

 matinic particles in the life-cycle of organisms gener- 

 ally. I will now deal with these points in order. 



The results obtained by physiological experiments 

 with regard to the functions of the nuclear and cyto- 

 plasmic constituents of the cell are now well known 

 and are cited in all the text-books. It is not neces- 

 sary, therefore, that I should discuss them in detail. 

 I content myself with quoting a competent and im- 

 partial summary of the results obtained : — 



"A fragment of a cell deprived of its nucleus may 

 live for a considerable time and manifest the power 

 of co-ordinated movements without perceptible im- 

 pairment. Such a mass of protoplasm is, however, 

 devoid of the powers of assimilation, growth, and 

 repair, and sooner or later dies. In other words, 

 those functions that involve destructive metabolism 

 may continue for a time in the absence of the nucleus ; 

 those that involve constructive metabolism cease with 

 its removal. There is, therefore, strong reason to 



NO. 2398, VOL. 96] 



believe that the nucleus plays an essential part in the 

 constructive metabolism of the cell, and through this 

 is especially concerned with the formative processes 

 involved in growth and development. For these and 

 many other reasons ... the nucleus is generally 

 regarded as a controlling centre of cell-activity, and 

 hence a primary factor in growth, development, and 

 the transmission of specific qualities from cell to cell, 

 and so from one generation to another.^ 



I have mentioned already in my introductory re- 

 marks that the only trustworthy test of chromatin is 

 its behaviour, and the whole of modern cytological in- 

 vestigation bears witness to the fact that the chro- 

 matinic particles exhibit the characteristic property 

 of living things generally, namely, individualisation 

 combined with specific behaviour. In every cell- 

 generation in the bodies of ordinary animals and plants 

 the chromatin-elements make their appearance in the 

 form of a group of chromosomes, not only constant in 

 number for each species, but often exhibiting such 

 definite characteristics of size and form, that par- 

 ticular, individual chromosomes can be recognised 

 and identified in each group throughout the whole 

 life-cycle. Each chromosome is to be regarded as 

 an aggregate composed of a series of minute chro- 

 matinic granules or chromioles, a point which I shall 

 discuss further presently. 



Even more remarkable than the relation of the 

 chromosomes to cell-reproduction is their behaviour 

 in relation to sexual phenomena. In the life-cycles 

 of Metazoa the sexual act consists of the fusion of 

 male and female pronuclei, each containing a definite 

 and specific number of chromosomes, the same 

 number usually, though not always, in each pro- 

 nucleus. It has been established in many cases, and 

 it is perhaps universally true, that in the act of 

 fertilisation the male and female chromosomes remain 

 perfectly distinct and separate in the synkaryon or 

 nucleus formed by the union of the two pronuclei, 

 and, moreover, that they continue to maintain and 

 to propagate their distinct individuality in every sub- 

 sequent cell-generation of the multicellular organism 

 produced as a result of the sexual act. In this way, 

 every cell of the body contains in its nucleus distinct 

 chromatinic elements which are derived from both 

 male and female parents and which maintain unim- 

 paired their distinct and specific individuality through 

 the entire life-cycle. This distinctness is apparent at 

 least in the germ-cell-cycle of the organism, but may 

 be obscured by secondary changes in the nuclei of the 

 specialised tissue-cells. 



Only in the very last stage of the life-cycle do the 

 group of male and female chromosomes modify their 

 behaviour in a most striking manner. In the final 

 generation of oogonia or spermatogonia, from which 

 arise the oocytes and spermatocytes which in their 

 turn produce the gamete-cells, it is observed that the 

 male and female chromosomes make a last appearance 

 in their full number, and then fuse in pairs, so as to 

 reduce the number of chromosomes to half that 

 previously present. 



As Vejdovsky has pointed out, there can be no more 

 striking evidence of the specific individuality of the 

 chromosomes than their fusion or copulation in rela- 

 tion to the sexual act. Is there any other constant 

 element or constituent of living organisms exhibiting 

 to anything like the same degree the essentially vital 

 characteristics of individuality manifested in specific 

 behaviour? If there is, it remains to be discovered. 



I come now to the question of the permanence and 



immortality, in the biological sense of the word, of 



I the chromatinic particles, which may be summarily 



stated as follows: the chromatinic particles are the only 



' 2 E. B. Wilson, " The Cell," second edition, 1911, pp. 30, 31. 



