THE MITOTIC CYCLE 



Cytochemical — Due to the prominence of the developments in this 

 subject in recent years, it is often thought of as the most modern branch 

 of cell biology, though its roots go back for more than a century, as 

 BakerI has made clear in an interesting historical survey. The develop- 

 ment of staining techniques in the later nineteenth century was pre- 

 ceded by Miescher's pioneer work on the nucleic acids, and for more 

 than a decade it was hoped that differences in the staining reactions of 

 cell components could be directly interpreted in chemical terms 

 (Hughes^), High hopes and sceptical reaction have marked the progress 

 of cytochemistry and even now the list of substances which can be 

 identified with reasonable probability within components of the cell 

 is still very small. In general, the difficulties increase as the localization 

 of smaller molecules and groups is attempted, and at present the identi- 

 fication of atoms other than by isotopic methods is virtually impossible. 

 Yet until methods are found adequate for tracing the movements 

 of calcium, for instance, within a cell during mitosis it will not be 

 possible to test so interesting a theory as that of Heilbrunn on the 

 nature of the mitotic stimulus. 



Although it would not be true to state that these four general headings 

 of research have been pursued in isolation from each other, this ten- 

 dency has been a strong one. The distribution of our present knowledge 

 in the subject is partly determined by the restriction of some lines of 

 inquiry to a few favourable sources of material. Thus much of what is 

 known about the cytoplasmic events in mitosis refers to marine eggs, 

 and the nature of the stimulus to enter division has mainly been studied 

 in tissue cultures. 



If evolutionary development had provided the Echinoderms with 

 a few large chromosomes, it is probable that our knowledge of the 

 nuclear and the cytoplasmic aspects of mitosis would be more coherent. 

 Schrader^ says that 'it is a striking fact that our knowledge of chromo- 

 some structure and behaviour is far in advance of what we know about 

 the rest of the mitotic figure', and although there has been recent pro- 

 gress in the study of the achromatic figure, it is certainly true that the 

 relations of the two sets of events is still largely obscure. We are, for 

 example, wholly ignorant at present of the nature of the actual stimulus 

 which sets oflf the movement of the chromosomes in anaphase. In 

 watching a living cell through mitosis, metaphase often passes rather 

 slowly, and one awaits the relief of the sudden and unheralded moment 

 when all the chromosomes move together. At such times, this question 

 suggests itself with peculiar insistence. In Michel's memorable film 

 of Orthopteran spermatogenesis, shortly before anaphase it can be seen 

 that the mitochondrial investment of the spindle bulges at the equator 

 towards the cell surface. Then anaphase begins, the mitochondria 

 retreat, and soon the cleavage furrow appears at the site where this 



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