October 14, 1915] 



NATURE 



191 



I distributed between the two daughter-cells, so that 



I for every chromatin-grain obtained by one daughter- 



I cell an exact counterpart is obtained by the other ; 



I in other words, of ensuring an exact qualitative, as 



' well as quantitative, partition of the chromatin- 



particles. In its perfect form this type of nuclear 



division is known as karyokinesis or mitosis, and all 



stages in its progressive development are to be found 



in the Protozoa. 



In the evolution of nuclear division by karyokinesis 

 two distinct processes are being developed and per- 

 il fected in a parallel manner, but more or less inde- 

 ' pendently ; first, the method of the partition and dis- 

 tribution of the chromatin-grains between the two 

 daughter-nuclei ; secondly, the mechanism whereby 

 the actual division of the nucleus and the separation 

 of the two daughter-nuclei are effected in the cell- 

 division. I have dealt elsewhere with the evolution 

 of the mechanism of karyokinesis as exemplified by 

 the numerous and varied types of the process found 

 amongst the Protozoa, and I need not discuss the 

 matter further here, but the behaviour of the chro- 

 matin-grains may be dealt with briefly. The main 

 feature in the process of the exact quantitative and 

 qualitative distribution of the daughter-chromatin 

 1) t ween the daughter-nuclei is the aggregation of the 

 cliromatin-grains or chromioles into definite, highly 

 individualised structures known as chromosomes. In 

 the most perfected forms of the process of chromo- 

 some-formation the chromioles become united into a 

 linear series termed by Vejdovsky a chromoneme, 

 which is supported upon a non-chromatinic basis or 

 .axis. 



The actual division of the chromatin takes place 

 by the longitudinal splitting of the chromoneme — in 

 other words, by simultaneous division into two of 

 each of the chromioles of which the thread is com- 

 posed. In this way every chromiole which was con- 

 tained in the original chromoneme is represented by 

 a daughter-chromiole in each of the two daughter- 

 chromonemes. It follows that the familiar process 

 of the splitting of the chromosomes in karyokinesis 

 is a mechanism which brings about in the most 

 >imple, sure and direct manner an exact quantitative 

 and qualitative partition of the chromatin-grains 

 between the two daughter-nuclei. 



The chromatin-cycle of a cell in which the process 

 of division by karyokinesis takes place in its most 

 p. rfectly developed form, may, therefore, be con- 

 ceived as follows. The nucleus in its resting state 

 contains a definite number of companies or brigades 

 of chromatinic units (chromioles), each brigade spread 

 over a certain extent of the nuclear framework form- 

 ing a karyomere. As a preparation to division each 

 separate brigade of chromioles falls into line as the 

 chromoneme, forming with its supporting substance 

 the chromosome; there are formed, therefore, just so 

 many chromosomes as there were karyomeres in the 

 nucleus. In this disciplined and orderly array each 

 chromiole undergoes its division into two daughter- 

 chromioles, so that each file or chromoneme of chro- 

 mioles splits into two files. At the reconstitution of 

 the daughter-nuclei each daughter-chromosome gives 

 rise to a karyomere again, the chromioles falling out 

 of the ranks and disposing themselves in an appar- 

 ently irregular manner on the newly built framework 

 of the daughter-nucleus to constitute their own par- 

 ticular karvomere. Thus karyokinesis differs only 

 from the most primitive method of division by chro- 

 midial fragmentation in that what was originally a 

 haphazard method of distribution has become a dis- 

 ciplined and orderly manoeuvre, performed with the 

 precision of the parade-ground, but in a space far 

 less than that of a nutshell. 



In the nuclear division of Protozoa, without going 

 into detail, it may be stated broadly that all stages 

 are to be found of the gradual evolution of the tac- 

 tical problem which constitutes karyokinesis. 



I have dealt briefly with the problem of the evolu- 

 tion of karyokinesis because the process of nuclear 

 division is, in my opinion, of enormous importance 

 in t^e general evolution of living organisms. I 

 have expressed elsewhere the opinion that the very 

 existence of multicellular organisms composed of 

 definite tissues is impossible until the process of 

 karyokinesis has been established and perfected. For 

 tissue-formation it is essential that all the cells which 

 build up any given tissue should be similar, practically 

 to the point of identity, in their qualities ; and if it 

 is the chromatin-elements of the cell which determine 

 its qualities and behaviour, then the exact qualitative 

 division of the chromatin, as effected in karyokinesis, 

 is indispensable as a preliminary to the production of 

 identically similar daughter-cells bv division of a 

 parent-cell. Hence it becomes intelligible why, 

 amongst Metazoa, we find the occurrence of nuclear 

 division by karyokinesis in its most perfect form to 

 be the rule, and "direct" division of the nucleus to 

 be the rare exception, while, on the other hand, in 

 the Protista, and especially in the Protozoa, we find 

 every possible stage in the gradual evolution of the 

 exact partition of the chromatin in the process of 

 nuclear division, from chromidial fragmentation or 

 the most typical amitosis up to processes of karyo- 

 kinesis as perfect as those of the Metazoa. 



I have confined myself to the evolution of the cell 

 as this organism is seen in its typical form in the 

 bodies of the multicellular organisms, starting from'^ 

 the simplest conceivable type of living being, so far 

 as present knowledge enables us to conceive it. But 

 there is not the slightest reason to suppose that the 

 evolution of the Protista took place only in the direc- 

 tion of the typical cell of the cytologist. Besides the 

 main current leading up to the typical cell, there were 

 certainly other currents tending in other directions 

 and leacling to types of structure very unlike the cells 

 composing the bodies of multicellular organisms. 



In this address I have set forth my conceptions of 

 the nature of the simplest forms of life and of the 

 course taken by the earliest stages of evolution, 

 striving all through to treaf the problem from a 

 strictly objective point of view, and avoiding as far as 

 possible the purely speculative and metaphysical ques- 

 tions which beset like pitfal's the path of those who 

 attack the problem of life and vitalism. I have, 

 therefore, refrained as far as possible from discussing 

 such indefinable abstractions as "living substance" 

 or "life," phrases to which no clear meaning can be 

 attached. 



How far my personal ideas may correspond to 

 objective truth I could not, of course, pretend to 

 judge. If I might be permitted to attempt an 

 impartial criticism of my own scheme, I think it 

 might be claimed that the various forms and types 

 of organisms in my evolutionary series, namely, the 

 simple cell or protocyte, the cytode or pseudomoneral 

 stage, the micrococcus, even the biococcus, are 

 founded on concrete evidence, and can be regarded as 

 tvpes actually existent in the present or past. On 

 the other hand, the rdle assigned by me to each type 

 in the pageant of evolution is naturally open to dis- 

 oute. For example, I agree with those who derive the 

 bacteria as primitive, truly non-cellular organisms, 

 directly from the biococcus through an ancestral 

 form, and not at all with those who would regard the 

 bacteria as degenerate or highly specialised cells. 

 I But the crux of my scheme is the homologv postulated 

 1 between the biococcus and the chromatinic particle 



NO. 2398, VOL. 96] 



