16 



THE CHROMOSOMES 



their number ; their points of attachment are ap- 

 proximately equally spaced round the edge, so that 

 if there are 6 chromosomes they will be 60° apart, 

 if there are 24 (as in Salamandra) they will be 15° 

 apart. Here it appears that the spindle elements 

 associated with the chromosomes form a circle round 

 the central element (Fig. 4a). In some organisms 



Fig. 4. — ' Polar views ' of chromosomes and spindle at meta- 

 phase ; a in the Salamander, where there are 24 large 

 chromosomes which arrange themselves on the periphery, 

 with a large central spindle element in the middle ; 

 h in an organism where there are 16 large chromosomes 

 and 16 microchromosomes which arrange themselves in 

 the centre of the spindle. 



the two chromatids at this stage are strictly parallel, 

 in others they wind round one another (cf. Fig. 56 

 and ^). 



Where there is no central element some of the 

 chromosomes may be entirely embedded in the 

 middle of the spindle. The following table shows 

 the number which usually occupy the central region.^' 



These arrangements are found when all the 

 chromosomes of the set are about the same size ; 



