30 THE CHROMOSOMES 



lapathum).^^* Between these limits nearly all pos- 

 sible numbers are found in at least one species of 

 animal or plant. Diploid numbers between 12 and 

 32 are common, those above and below these figures 

 being progresgively rarer. Both in animals and 

 plants the commonest diploid number is 24 (Tables 

 VIII and IX). 



As regards size, the smallest known chromosomes 

 are approximately 0-25 ju in length and about the 

 same breadth at the metaphase of mitosis ^'"^ ; the 

 longest ones are about 25 ju long and 2 // in width. ^ 



Normally each chromosome possesses only a single 

 spindle attachment. By irradiation with X-rays it 

 is, however, possible to cause two chromosomes to 

 fuse in such a way that a compound chromosome is 

 formed with two spindle attachments (Fig. 6a and 6). 

 Such a chromosome may behave in either of two ways 

 at mitosis ^^^ ; either both spindle attachments in 

 each chromatid may go to the same pole at anaphase 

 or to opposite poles (if those in one chromatid go to 

 opposite poles, naturally those in the other will do 

 likewise). When the spindle attachments in a 

 chromatid go to opposite poles the part of the chroma- 

 tid between them will be stretched out and eventually 

 broken. Apparently each of the two ways of divi- 

 sion happens in 50 per cent of cases ; the number of 

 chromosomes with two spindle attachments is thus 

 progressively reduced in the course of a few divisions, 

 and such chromosomes stand no chance of becoming 

 permanent. 



One case exists, however, in which normal chromo- 

 somes have more than one spindle attachment each. 

 That is in Ascaris megahcephala where the middle 

 region of the long chromosomes found in the ' germ- 

 line ' appears to contain about sixteen separate 

 spindle attachments (Fig. 6c). Owing to the fact 

 that these are very close together all those in one 



* Some Protozoa (e.g. Aggregaia) appear to have even 

 more than this. 



