1190 BIOLOGICAL EFFECTS OF RADIATION 



positions, so that not only the different chromosomes but even their 

 rather minute sections can be recognized and identified. 



In Drosophila, all the chromosomes of a nucleus become attached 

 together at the spindle fibers to the so-called chromocenter — a foamy 

 mass showing no clear cross striations, and formed by an involution of 

 the inert regions of all the chromosomes (Heitz, 49, 50; Painter, 95, 96). 

 Moreover, in the salivary glands of the grown-up larvae and young pupae 

 the homologous chromosomes undergo a process of very intimate pairing 

 which, at least superficially, resembles the meiotic pairing. The pairing 

 is remarkably accurate so that the homologous bands are closely apposed 

 to each other. This pairing is especially advantageous for studying 

 chromosome rearrangements, since it enables one to determine the loca- 

 tion of breaks and reattachments with a hitherto undreamed degree of 

 accuracy (Painter, 96, 97). 



The relative distances between various genes on the cytological maps 

 built on the basis of the salivary gland chromosomes are sometimes differ- 

 ent from those on the ordinary cytological maps (Painter, 97). This 

 is due primarily to the practical disappearance of the inert regions of 

 the chromosomes which are included in the chromocenter in the salivary- 

 gland cells, but also to some other differences thus far not understood. 



The possibilities open by the salivary-gland-chromosome method can- 

 not be overestimated, as shown already by the results of Painter and 

 others. Very small deficiencies, duplications, and inversions can be 

 detected, and their extent can be studied. It is tempting to suppose that 

 each of the disks composing the salivary-gland chromosomes has a definite 

 relationship to a single gene. Even if this supposition proves to be far 

 too crude, the problem of the possible effect of the apparent and real 

 point mutations on the cytologically visible chromosomes can at last be 

 approached. Finally, a comparison of the salivary-gland chromosomes 

 in closely related species may give some much needed information on the 

 nature of specific differences. 



CYTOLOGICAL DEMONSTRATION OF CROSSING OVER 



Studies on cytological maps have afforded conclusive proof of the 

 theory of the linear arrangement of genes within the chromosomes. A3 

 is well known, this theory has been arrived at by Morgan on the basis 

 of studies on crossing over in Drosophila. Morgan assumed that genes 

 are arranged in a definite linear order, and that this order is preserved 

 during the entire life cycle of the cell. At gametogenesis, however, 

 the homologous chromosomes exchange segments, and this process of 

 segmental interchange has its genetic expression in the variable degrees 

 of linkage exhibited by various genes. Although the whole development 

 of genetics since the time when Morgan advanced this theory has justified 



