6 CYTOLOGICAL TECHNIQUE 



segments or karyosomes, do not undergo the same telophase 

 transformations as the rest, but remain as thick, somewhat 

 irregular bodies, easily visible in the living cell. The hetero- 

 chromatic segments are colourable without difficulty in fixed 

 preparations, since they contain deoxypentosenucleic acid (DNA), 

 which is the main substance responsible for the fact that the 

 mitotic chromosomes are so easily dyed. DNA has a strong 

 affinity for all members of the large group of colouring agents 

 known as basic dyes (p. 90). 



DNA is not confined to the heterochromatic segments, but 

 appears to be distributed throughout the whole of the rest of the 

 nucleus (apart from the nucleolus), at a lower concentration than 

 in the heterochromatic segments. ^^^ The form in which it is 

 distributed is not known. Possibly it is strung out along the fine 

 threads that constitute the normal or euchromatic segments of 

 the chromosomes; possibly these threads are at this stage com- 

 posed of protein only, and the DNA is in colloidal solution in the 

 nuclear sap. 



Before the chromosomes reappear distinctly in early prophase, 

 they have split longitudinally throughout their length. It is not 

 known exactly when this splitting occurred, and an arbitrary 

 decision must therefore be reached as to whether the chromosomes 

 should be represented in a diagram as split or unsplit. They are 

 represented in fig. 1 as single threads, swollen in particular places 

 into heterochromatic segments. Only two pairs of chromosomes 

 are represented in the diagram, but in the great majority of 

 animals the diploid number exceeds ten. 



In dyed preparations of cells undergoing mitosis, each chromo- 

 some appears to be interrupted at a particular spot, where there 

 is a short segment lacking affinity for basic dyes. This segment is 

 the spindle-attachment or centromere. Long chromosomes are 

 attached to the spindle by this body, but short ones are wholly 

 embedded in the spindle. The centromere is the part of the 

 chromosome that leads the rest in the movement towards the 

 centriole in early anaphase. Since each centromere occupies an 

 unvarying position in each chromosome, it presumably persists 

 from telophase to the succeeding prophase. 



