THE MITOTIC CYCLE 



Apart from the difficulty of demonstrating direct nuclear division, 

 the main objection to admitting that it occurs rests on the question of 

 how far a cell is viable if the nucleus does not contain a full comple- 

 ment of chromosomes. It is inconceivable that the complex procedure 

 of mitosis whereby two equal sets of chromosomes are formed and dis- 

 tributed to two equivalent daughter nuclei can also take place in a 

 concealed fashion during a simple constriction of the nucleus into two 

 portions. Yet the mammalian erythrocyte shows that some cells can 

 exist even without any nuclear component; generally, however, cir- 

 culatory cells have a limited life and no further potentialities of develop- 

 ment. If amitosis can occur in embryonic tissues, then the possibility 

 must be admitted that cells arising in such a manner can beget a 

 persistent lineage of other cells, which might even contribute to the 

 germ line. Among Western biologists, the evidence for this is not con- 

 sidered sufficient to overthrow the chromosome theory of heredity. 

 Nevertheless, rather than brush aside all observations of direct nuclear 

 division with a theoretical objection however strong, we should 

 address ourselves to further consideration of the problem of what 

 deviation from the normal complement of chromosomes, or 'aneuploidy' 

 as BiESELE^^' terms it, can be tolerated by different cells during the 

 various stages of the life cycle. Early embryos soon deviate from normal 

 development when the full complement of chromosomes are not present 

 in the nucleus (Poulson^°°), but there is increasing evidence that 

 irregular numbers of chromosomes may be found in nuclei at later 

 stages of development (Timonen and Thermann^"^) . If more were 

 known on this topic, the problem of amitotic division could be more 

 surely approached. 



REFERENCES 



Somatic prophase and telophase in the living cell 



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