THE MITOTIC CYCLE 



of this theory is derived from the comparative effects of X-rays on cells 

 at various times before prophase (Lea^^°). A break in an unsplit 

 chromosome will result in similar effects in both chromatids, while 

 unequal breakages will denote that the chromatids were already sep- 

 arate at the time of irradiation. It is found that the proportion of the 

 former gradually decreases as mitosis approaches. The question at issue 

 is really one of the order of size of the chromosomal units at the time 

 when they part into two groups. The verdict of Lea^^^ on this question 

 is that Tn view of the smooth rather than discontinuous change from 

 the stage at which isochromatid breaks are much less frequent than 

 chromatid breaks, it is evident that the X-ray method cannot give 

 conclusive evidence of a chromosome being unsplit'. 



Spirals 



Wherever the individual chromonemata within a chromosome are 

 separately distinct, they are seen to be spirally coiled. Often special 

 technical methods, such as 'pretreatment' before fixation, are needed 

 to reveal the spirals, but they are visible in some living pollen mother 

 cells, and in such were first described by Baranetsky.^^i xhe phase 

 microscope clearly reveals the structure of the metaphase chromosomes 

 in the living pollen mother cells of Tradescantia (Ruch^^^). The whole 

 subject has recently been admirably reviewed by Manton.^^^ She has 

 suggested that spirals result from a multiple stranded condition of 

 the chromosomes (Manton^^*). 



When a chromosome is made up of a number of closely apposed spiral 

 chromonemata an extremely complex appearance results, in which 

 the individual spirals lie near the limits of microscopical resolution. It 

 is therefore not surprising that observers disagree whether a further 

 minor coil exists within the major spiral. Nebel^^^ has attempted to 

 analyse the structure of intact chromosomes by comparing them with 

 macroscopic models made of glass spirals. Such a comparison, however, 

 neglects the effects of diffraction, which may well play some part in 

 the make-up of the magnified image of so complex an object. 



Much attention has been given to the question whether there are 

 regularities in the directions of the spirals of the chromonemata, and 

 most observers now agree that left- and right-handed spirals occur 

 equally at random. White^^o j^^s shown that in a group of grasshopper 

 spermatogonia derived from a single mother cell, the number of right- 

 and left-handed chromosomes is approximately equal. Manton and 

 Smiles127 j^ayg shown in their superb ultraviolet photomicrographs of 

 the chromosomes of the fern Osmunda that the two chromonemata of 

 a prophase chromosome may actually be coiled in opposite directions. 



It is only in a small minority of animal cells that spiral structure is 

 clearly seen, as for instance in the Orthopteran chromosomes 



94 



