Cell Division 



119 



the degree of crossing over, certain segments 

 of the homologous chromosomes. The second 

 division proceeds without chromosome re- 

 production. The kinetochores orienting on 

 the spindle are therefore the sister kineto- 

 chores of the first division. The fundamental 

 features of meiosis are thus pairing in the 

 first division followed by a separation of 

 the four chromatids in each chromosome 

 pair by two karyokineses without any further 

 chromosome reproduction. The four nuclei 

 resulting from these divisions are therefore 

 haploid. 



A somewhat different way in distributing 

 the four chromatids of each bivalent during 

 the two meiotic divisions has been described 

 in some insects with diffuse kinetochore. It 

 was discussed by Ris ('42), Oksala ('43) and 

 Hughes-Schrader ('48). 



AMITOSIS 



According to Remak the nucleus divides by 

 simple constriction. After the discovery of 

 mitosis many cytologists continvied to believe 

 that such direct division, or "amitosis" as 

 Flemming called it, was an alternate method 

 for nuclear and cell reproduction. With the 

 establishment of the chromosome theory of 

 inheritance it became very improbable that 

 cells could proliferate by amitosis. A critical 

 study of most cases of amitosis disclosed that 

 they were either modified or abnormal mi- 

 toses (pseudo-amitoses, cf. Politzer, '34) or 

 based on faulty observations. There remained, 

 however, a number of observations of con- 

 striction and fragmentation of nuclei into 

 two or more parts, especially in certain 

 highly specialized tissues that no longer 

 divide mitotically. Only very rarely is nu- 

 clear fragmentation followed by division of 

 the cytoplasm (Schrader, '45), and then 

 there is no evidence that these cells persist 

 or are able to multiply. Nuclear fragmenta- 

 tion is especially common in cells that have 

 become polyploid throvigh endomitosis (Heid- 

 enhain, '19) or inhibited mitosis (Pfuhl, '39; 

 Bucher, '47). It is, however, never followed 

 by cell division. 



Direct division of the nvicleus, or "ami- 

 tosis," is not a method of cellvdar prolifera- 

 tion but of nuclear fragmentation, generally 

 in polyploid cells that are no longer able to 

 divide. As suggested already by Flemming, 

 it appears to be related to increased physio- 

 logical activity of a cell. Especially in large 

 polyploid cells it would result in a better 

 distribution of nuclear material throughout 

 the cytoplasm. 



The term "amitosis" is misleading, since it 

 suggests an alternative to mitosis. Reproduc- 

 tion of cells can take place only through mi- 

 tosis. Reproduction of nuclear material within 

 a cell may occur through mitosis or endo- 

 mitosis. The endomitotically enlarged nu- 

 cleus may remain single or break up into two 

 or more fragments. I would suggest that 

 this process be called "nuclear fragmenta- 

 tion" and that the term "amitosis" be elim- 

 inated. 



REFERENCES 



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Annau, E., Manginelli, A., and Roth, A. 1951 In- 

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Auerbach, E., and Doljanski, L. 1945 Effect of 

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and King, R. L. 1936 The effect of ul- 



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