18 THE CHROMOSOMES 



embedded in the substance of the spindle, they can 

 be seen to have spindle attachments of exactly the 

 same nature as the peripheral ones. 



Where a chromosome has been broken into two 

 parts as a result of irradiation by X-rays that part 

 which contains the spindle attachment becomes 

 associated with the developing spindle at prometa- 

 phase, while the part lacking a spindle attachment 

 floats freely in the cytoplasm and never becomes 

 attached to the spindle. ^^^' ^^^ There is thus some 

 evidence for regarding the spindle attachment as 

 the only part of the chromosome which plays a part 

 in organizing the gelation of the spindle elements 

 from the original nuclear sap ; perhaps ' spindle - 

 element-organizer ' would be a clumsy but descriptive 

 name for it. 



The position of the spindle attachment is constant 

 for each individual chromosome, but may vary from 

 one chromosome to another in the set. Thus in 

 Drosophila melanogaster (Fig. 20a) chromosomes I 

 and IV have subterminal spindle attachments, while 

 chromosomes II and III have median attachments. 

 Where the attachment is median the chromosome 

 will have the shape of a V with two limbs of equal 

 length ; where it is submedian the two limbs will 

 be unequal (Fig. 5). It was formerly believed that 

 the spindle attachment was terminal in many cases 

 and a distinction was drawn between ' V-shaped ' 

 and ' rod-shaped ' chromosomes. It is now known 

 that the attachment is never quite terminal ; in 

 other words there are always two limbs to the V, 

 only one may be so short as to be practically below 

 the limit of optical resolution. i*^' i^^' ^^^ 



In many cases of chromosomes with median or 

 submedian spindle attachments there appears to be 

 a minute granule in the centre of the spindle attach- 

 ment which stains with aniline dyes and Haema- 

 toxylon ; it resembles the minute granule which 

 forms the short limb of ' rod-shaped ' chromosomes. 



