334 GENERAL DISCUSSION 



It is difficult to recognize what is the biochemical process and at what 

 time mitosis is initiated. 



In the first place, it is necessary to look into the interconnections 

 of the biosynthesis of protein, RNA and DNA. This is connected with 

 the problem of RNA division between two daughter cells when the 

 nucleolus dissolves (given the dissolution of the nucleolus). The forma- 

 tion of the spindle has also up to now not been sufficiently studied. 



We do not as yet know what takes j^lace in the formation of the 

 sjHudle. Is this protein reorganized or synthesized? 



And finally, it is necessary to study the connection between cyto- 

 plasmic processes and nuclear processes. In certain cases, the cyto- 

 plasm may exist and even divide in the absence of a nucleus. Thus the 

 inhibition of division of the cytoplasm may depend upon the organelles 

 of the cytoplasm. 



belgovskaya: The synthesis of DNA and mitosis, and even such a 

 simple means of cell division as amitosis, are not necessarily connected. 



After the irradiation of Actinomycetes the synthesis of the DNA is 

 rapidly I'estored, but the division of the nuclei is not restored. Some- 

 times nuclei reach colossal dimensions and contain an enormous 

 amount of DNx4, but they do not divide. 



The mechanism of mitosis is indissolubly connected with cyto- 

 plasmic structures, as Errera has said. But we have, however, every 

 reason to suppose that the functions of these cytoplasmic structui'es 

 are under the control of the cell nucleus. The following facts testify to 

 this. The cleavage of fish eggs containing nuclei iiTadiated with a dose of 

 40 r proceeds normally up to the late blastula stage, while the cells 

 contain the products which precede the activity of the nucleus. But 

 when these jiroducts are exhausted, mitosis in the cells quickly ceases 

 and the cells perish. 



HOLMES : It is known that mitosis can be inhibited by means of a mecha- 

 nism not connected with the inhibition of DNA synthesis. Small doses 

 of X-rays stop cells going into mitosis even when the irradiation is 

 carried out after the completion of DNA synthesis in those cells. We 

 shall illustrate this situation by reference to the regenerating liver of 

 rats. When proliferating tissue is irradiated with 450 r the advance into 

 mitosis of all the cells is rapidly inhibited but I do not know the duration 

 of this effect. 



On the other hand, a 450 r dose given to the cells before the beginning 

 of the DNA synthesis caused a delay of the beginning of the synthesis 

 amounting to 12 hr, but did not alter the form of the DNA synthesis 

 curve, nor did it alter the time relationship between the peak of the 



