6 THE CHROMOSOMES 



effective. In Fishes and Amphibia prolonged starva- 

 tion followed by a meal may cause a considerable 

 increase in the number of dividing nuclei, while in 

 other cases starvation alone may be sufficient. ^^^ 

 Such diverse stimulants as a peritoneal in j taction of 

 foreign blood serum *^ or even a bacillus culture ^^ 

 have been found to produce the same effect. What 

 the underlying chemical mechanism is in these cases 

 we have no idea. It must be pointed out that a mere 

 increase in the number of dividuig nuclei in a histo- 

 logical section is not sufficient to j)r()ve that mitosis 

 has been stimulated hi resting nuclei ; it may e(jually 

 well result from a slowing down of mitosis. 



Gross mechanical injury to the nuchuis such as 

 results from the withdrawal of a microdissection 

 needle i)reviously inserted into the* nucleus, will often 

 produce a sudden acceleration of mitosis, ^^ but in 

 some cases it will cause the normal course of mitosis 

 to be reversed, so that nuclei which have already 

 entered on the division cycle go back into the resting 

 stage. 1^* 



The effects of irradiation with X-rays also vary 

 from one type of tissue to another. In some cases 

 mitosis ceases altogether in a tissue for some days 

 after irradiation ; cells which would have entered on 

 mitosis are retarded in the resting stage. ^^'^ Jn 

 other cases X-rays probably accelerate the onset of 

 mitosis. The whole subject of nuclear pathology is 

 urgently in need of re-interpretation. Thus many 

 nuclei when injured or dying become 'i)ycn(Aic\ 

 that is to say their chromosomes become fused into 

 a single large mass which stahis intensely with dyes 

 like Haematoxylin and anilin derivatives. Pycnosis 

 is probably to be interpreted as a highly modified 

 non-f\mctional mitosis, since in some cases ^^'' ])yc- 

 notic nuclei may begin to divide, but we do not 

 know what actually happens to the chromosomes in 

 a pycnotic nucleus. 



Some recent authors have omitted the primary dis- 



