No. 593] THE EVOLUTION OF THE CELL 



275 



newly-built framework of the daughter-nucleus to consti- 

 tute their own particular 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 disci- 

 plined and orderly manoeuvre, performed with the preci- 

 sion 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 evolution of the tactical 

 problem which constitutes karyokinesis. The chromo- 

 somes in the more primitive types of nuclear division are 

 usually very numerous, small, irregular in number and 

 variable in size ; the splitting of the chromosomes is often 

 irregular and not always definitely longitudinal ; and dis- 

 tinct karyomeres have not so far been recognized in the 

 nuclei of Protozoa. In many cases only a part, if any, of 

 the chromatin falls in to form the chromosomes, 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 pro- 

 mitosis, mesomitosis and metamitosis, for detailed ac- 

 counts 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 importance in the general 

 evolution of living organisms. I have expressed else- 

 where 29 the opinion that the very existence of multicellu- 

 lar 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 simi- 

 lar, practically to the point of identity, in their qualities ; 

 and if it is the chromatin^elements of the cell which deter- 

 mine its qualities and behavior, then the exact qualitative 



29 Op. cit., p. 120. 



