400 TRANSACTIONS OF SECTION D. 



that the familiar process of the splitting oi the chromosomes in karyokinesis 

 is a mechanism which brings about in the most simple, sure and direct manner 

 an exact quantitative and qualitative partition of the chromatin-grains between 

 tlie two daughter-nuclei. In the sequel each daughter-nucleus is built up, 

 according to Vejdovsky, entirely and solely from one of the two daughtei- 

 elumps of chromosomes, and each chromosome is resolved again into its con- 

 stituent ehromioles, giving rise in some cases to a definite portion of the 

 nucleus, a karyomere, from which again, at the next nuclear division, the 

 chromosome is reconstituted by the ehromioles falling into line in an orderly 

 manner. 



The chromatin-cycle of a cell in which the process of division by karyoki- 

 nesis takes place in its most perfectly developed form, may, therefore, be 

 conceived as follows. The nucleus in its resting state contains a definite 

 nimiber of companies or brigades of chromatinic units (ehromioles), each 

 brigade spread over a certain extent of the nuclear framework forming ,i 

 karyomere. As a preparation to division each separate brigade of ehromioles 

 falls into line as the rhromoneme, 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 

 ihromiole undergoes its division into two daughter-chromioles, so that each 

 file or chromoneme of ehromioles splits into two files. At the reconstitution 

 of the daughter-nuclei each daughter-chromosome gives rise to a karyomere 

 again, the ehromioles falling out of the ranks and disposing themselves in an 

 apparently irregular manner on the newly-built framework of the daughter- 

 nucleus to constitute their own parti<ular karyomere. Thus karyokinesis 

 differs only from the most primitive method of division by chromidial 

 fragmentation in that what was originally a haphazard method of distribution 

 has become a disciplined 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 excessive detail, 

 it may be stated broadly that all stages are to be found of the gradual evolu- 

 tion^ of the tactical problem which constitutes karyokinesis. The chromosomes 

 in the more primitive types of nuclear division are usually very numerous, 

 small, irregular in number and variable in size ; the splitting of the chromo- 

 somes is often irregular and not always definitely longitudinal ; ahd distinct 

 karyomeres have not so far been recognised in the nuclei of Protozoa. In 

 many cases only a part, if any, of the chromatin falls in to form the chromo- 

 somes, and a greater or less amount of it remains in the karyosome, which 

 divides directly into two. The various types of nuclear division in Protozoa 

 have been classified as promitosis, mesomitosis and metamitosis, for detailed 

 accounts of which those interested must refer to the text-books and original 

 descriptions. 



I have dealt briefly with the problem of the evolution of karyokinesis 

 because the process of nuclear division is, in my opinion, of enormous import- 

 ance in the general evolution of living organisms. I have expressed else- 

 where "' 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 ehromatin-elements of the 

 cell which determine its qualities and behaviour, then the exact qualitative 

 division of the chixjmatin, as effected in karyokinesis, is indispensable as a 

 preliminary to the production of identically-similar daughter-cells by 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 karyokinesis as perfect as those 

 of the Metazoa. 



" Op. cit. p. lao. 



