36 THE CHROMOSOMES 



results in part from a fusing together of the spindle 

 attachments and adjacent regions of all the 

 chromosomes. 



The nuclei of the salivary gland cells are very large 

 (about 25 /i in diameter in Drosophila) but even so 

 the chromosomes are so long that they are tangled 

 and coiled up so as to pack them inside the nuclear 

 membrane. The usual method of studying these 

 chromosomes consists in fixing and staining them 

 simultaneously with a saturated solution of carmine 

 in 40 per cent acetic acid to which a trace of an iron 

 salt is added just before use. The preparations are 

 then lightly crushed under a cover-glass in order to 

 rupture the nuclear membranes and spread out the 

 chromosomes on the slide. This crushing results in 

 a considerable stretching of the chromosomes ; the 

 X-chromosome of Z). melanogaster maybe 260// long, 

 in acetocarmine preparations, while in the living 

 salivary gland nucleus it is probably only about 150^ 

 in length. Even so the unst retched chromosomes in 

 the salivary gland chromosomes are at least 50 times 

 as long as the chromosomes at ordinary somatic 

 mitoses and 1000 times their volume. 



Each of the homologous chromosomes which pair 

 to form the threads in the salivary gland nucleus is 

 transversely striated with bands which stain with 

 ordinary nuclear dyes (e.g. Crystal Violet and 

 Haematoxylin). These cross bands are of varying 

 thickness and are separated by non-staining inter- 

 nodes. It is apparently * these internodes which 

 stretch when the nucleus is crushed. The bands 

 correspond exactly in position in the two homologous 

 chromosomes and are always the same for a particular 

 chromosome in different individuals of the same 

 species. The thicker bands as seen in the uncrushed 

 nuclei are apparently made up of several thinner 

 bands which can be separated by crushing and stretch- 

 ing under a cover -glass. The total number of bands 

 in the X-chromosome of D. melanogaster is at least 



