NUCLEI AND KINETIC ELEMENTS 113 



acteristic division figure representing the most highly perfected 

 mechanism for the equal distribution of the more important cell 

 elements which are thus perpetuated from generation to generation 

 by equal division. Such a perfected mechanism, termed a karyo- 

 kinetic or mitotic figure, is characteristic of nuclear division in cells 

 of the Metazoa and of higher plants, the combination of processes 

 whereby the constituent parts are equally distributed to daughter 

 cells being known as indirect division, karyokinesis, or mitosis. 

 Such processes in\^olve division of centrioles and centrosomes, forma- 

 tion of a fibrillar spindle figure, dissolution of the nuclear membrane, 

 aggregation of chromomeres into compact chromosomes which are 

 identical in size, shape and number in corresponding cells of all 

 individuals of the same species, and the longitudinal division of each 

 chromosome in all somatic cells, separation of the daughter chromo- 

 somes and reconstruction of the daughter nuclei. In all Metazoa 

 the processes of mitosis differ only in minor details and mitosis is 

 the characteristic type of nuclear division, although direct division, 

 whereby the nucleus divides without the formality of centrosomes 

 and spindle or chromosome formation is known in a few cases. 



In Protozoa, on the other hand, there is no one t^-pe of nuclear 

 division common to all forms. Here we find gradation, in the asso- 

 ciation of constituent nuclear and cytoplasmic kinetic elements 

 during division resulting in an enormous variety of division types. 

 These vary in complexity from a simple dividing granule to mitotic 

 figures as elaborate as in the tissue cells of higher animals and plants. 

 Some observers see in these diverse t;^^es a possible evolution of the 

 mitotic figure of IMetazoa and use them as one would use the sep- 

 arate pieces of a picture puzzle to reconstruct its past history in 

 development. Terms like "promitosis" (Naegler), "mesomitosis" 

 (Chatton) and "metamitosis" (Chatton) may serve a useful purpose 

 to indicate general t^^pes of the association of nuclear and cjto- 

 plasmic elements during division, but when an efi'ort is made to give 

 a specific name to each step in an increasingly complex series the 

 result is a confusion of terms which defeats the useful purpose 

 intended. Thus Alexeieft' proposes a large number of specific 

 names, not all his own, it is true, for protozoon division t\pes which 

 he regards as sufficiently definite to permit of recognition.* 



Because of the multitude of diverse types of division figures in the 

 Protozoa the difficulty of treating them in any general way has been 

 admitted by all students of etiology as well as by protozoologists. 

 I shall endeavor here to convey an idea of this diversity and at the 

 same time to describe some of the more frequent types of division 

 figure without confusing the issue still more by my own views as to 



* These terms include Promitosis, Proteromitosis, Haplomitosis, Cryptohap- 

 lomitosis, Eurypanmitosis, Cyclomitosis or Polymitosis, Polyrheomitosis, 

 Metamitosis, etc. 



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