ANAPHASE SEPARATION 29 



This body is about 0*2 fi in diameter, in the largest chromosomes. 

 It has received various names according to the stage or organism 

 in which it has been seen. We shall find later that the centromere 

 has important functions in the movements of chromosomes quite 

 apart from the spindle, and since the same movements occur even 

 when it is not visible with the treatment used, it seems necessary 

 to assume that it is always present, but that, like the centrosomes, 

 it is of variable staining capacity and in the smaller chromosomes 

 is too small to see. The chromatids remain in contact at their ends 

 until the anaphase separation finally draws them apart. It is 

 common to refer to the separation of the chromatids as the 

 ," division " of the chromosome, but, as we have already seen, the 

 division has taken place much earlier, and, throughout the prophase, 

 chromatids have been merely held together either by a common 

 pellicle surrounding them or, more probably, by a specific attraction 

 existing between them. The only part of the chromosome that 

 divides at this stage is the centromere, and when its division is 

 inhibited artificially the division of the nucleus is itself arrested at 

 metaphase {cf. Ch. X). At anaphase therefore it is an attraction 

 which has to be overcome rather than an intricate connection 

 which has to be severed. The relations of the chromatids at meiosis 

 cannot be understood unless this principle is grasped. 



When the separation is complete the corresponding groups of 

 chromatids, now called " daughter chromosomes," pass towards 

 opposite poles. This movement has been analysed by Belar with 

 remarkable success, working with living cells (1927, 1929, a and 6). 

 In the first part of the movement the spindle undergoes no important 

 change in shape or character, and it appears that the movement is 

 independent of external variations that can be brought to bear 

 on it experimentally. So far it may be said to be " autonomous." 

 At late anaphase the situation is entirely different. There is little 

 evidence that the chromosomes move any further through the 

 spindle, but the spindle itself changes shape. The middle part, 

 between the separated bodies of chromosomes, expands lengthwise 

 and contracts correspondingly crosswise so that the chromosomes 

 are pushed apart and the anaphase movement completed. Belar 

 maintains that this is an inherent and " active " property of the 



