452 TRANSACTIONS OF SECTION D. 



' 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 

 impairment. 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 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 may add here that the results of the study of life-cycles of Protozoa are 

 entirely in harmony with this conceptioni of the relative importance of nuclear 

 — that is chromatinic — ^and cytoplasmic cell-constituents, since it is not 

 infrequent that in certain phases of the life-cycle, especially in the microgamete- 

 stages, the cytoplasm is reduced, apparently, to the vanishing point, and the 

 body consists solely of chromatin, eo far as can be made out. In not one 

 single instance, however, has it been found as yet that any normal stage in 

 the developmental cycle of organisms consists solely of cytoplasm without any 

 particles of chromatin. 



While on the subject of physiological experiment, there is one point to which 

 I may refer. Experiments so far have been carried on with Protozoa possessing 

 definite nuclei. It is very desirable that similar experiments should be con- 

 ducted with forms possessing chromidia in addition to nuclei, in order to test 

 the physiological capabilities of chromatin-particles not concentrated or orga- 

 nised. Arcella would appear to be a very suitable form for such investigations. 

 This is a point to which my attention was drawn by my late friend Mr. C. H. 

 Martin, who has lost his life in his country's service. 



I have mentioned already in my introductory remarks that tlie only 

 reliable test of chromatin in its behaviour, and the whole of modern cytological 

 investigation bears witness to the fact that the chromatinic particles exhib't 

 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 aippearance 

 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 

 particular, individual chromosomes can be recognised and identified in each 

 grouip throughout the whole life-cycle. Each chromosome is to be regarded as 

 an aggregate composed of a series of minute chromatinic granules or chromioles, 

 a point which I shall discuss further presently. Most striking examples of the 

 individualisation of chromosomes have been made known recently by Dobell 

 and Jameson -° in Protozoa. Thus in the Coccidian genus Aggregata six 

 chromosomes appear at every cell-generation, each differing constantly in 

 length if in the extended form, or in bulk if in the contracted form, so that 

 each of the six chromosomes can be recognised and denoted by one of the 

 letters a to / at each appearance, a being the longest and / the shortest. 



Even more remarkable than the relation of the chromosome to cell-reproduc- 

 tion is their behaviour in relation to sexual phenomena. In the life-cycles of 

 Metazoa the sexual act consists of the fusion of male and fem.ale pronuclei, 

 each containing a definite and specific number of chromosomes, the same 

 number usually, though not always, in each pronucleus. It has been estab- 

 lished 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 syrikaryon 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 subsequent 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 



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

 =" Proc. Boy. Soc. (B), vol. 89. {In the Press.) 



