MITOSIS AND AMITOSIS. 



EDWIN G. CONKLIN, 

 PRINCETON UNIVERSITY. 



It was once supposed that all forms of nuclear division were 

 of the type which is now called amitosis in which the nucleus 

 undergoes simple constriction. After the discovery of the 

 complicated method of nuclear division now known as mitosis 

 and the recognition of its very general occurrence in plants and 

 animals, the doubt was expressed whether amitosis ever occurs 

 as a normal process; its occurrence in pathological and degen- 

 erating cells is undoubted. However, the number of cases in 

 which nuclear constriction is known to occur regularly in certain 

 tissue cells is too great to warrant the belief that it is always a 

 pathological phenomenon. 



The only question of present importance in connection with 

 this form of nuclear division is as to its bearing on the chromo- 

 some theory of heredity. If amitotic division of the nucleus, 

 followed by division of the cell body, ever occurs in the germ 

 cell cycle or in cleavage cells undergoing normal differentiation 

 it would go far to disprove the "individuality" or persistent 

 identity of chromosomes and consequently the chromosome 

 theory of inheritance, since it is scarcely possible that individual 

 chromosomes can be equally divided and their halves accurately 

 distributed to daughter cells by the process of amitosis. The 

 majority of cytologists at the present day concur in the opinion 

 that amitosis does not normally occur in embryonic or germ 

 cells and that when it does occur in such cells the ensuing de- 

 velopment is abnormal. On the other hand some investigators 

 maintain that amitosis occurs regularly and predominantly in 

 the genesis of the germ cells and also in the blastomeres and 

 embryonic cells of several species of animals and plants and for 

 this reason among others they reject the chromosome theory. 

 Child in particular has maintained this position in a series of 

 papers (1907-1911) dealing primarily with the cestocle Monezia, 



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