HETEROCHROMA TIN 307 



6. Chemical Tests. 



Modilewski, 1932. Neottia, Galtonia, Allium (two types of nucleolus). 



Bauer, 1933. Oocytes of insects. 



Gardiner, 1935. Tenehrio. 



Geitler, 1935 a and h. Spirogyra spp. 



(iii) Differential Cycles of Condensation. Correlated with the 

 cycle of coiling and uncoiling which the chromosomes undergo 

 during mitosis and meiosis there is also a cycle of changes in fixation- 

 staining reactions in the chromosomes. This cycle is not under- 

 stood in physical terms, but it is recognised as being characteristic 

 for particular nuclei at particular stages of the life-cycle and sub- 

 jected to particular treatments. It consists in a change from 

 deeply-staining metaphase chromosomes to lightly-staining or non- 

 staining resting stage chromosomes. The behaviour of particular 

 chromosomes or parts of chromosomes has long been known to be 

 different in this respect from that of the rest of the complement, e.g., 

 Rosenberg's prochromosomes (1909) at mitosis in plants and the 

 differentially condensed chromosomes at meiosis in many animals. 

 Recently, owing largely to the work of Heitz, differences have been 

 recognised at mitosis in both plants and animals to which a genetic 

 foundation may be ascribed. The difficulty, however, in establish- 

 ing this relationship lies in the great variability of the behaviour 

 of differential parts of chromosomes in different organisms, even 

 as seen under the same conditions of treatment, and the still greater 

 variability with different treatment. 



Broadly we may say that there are three types of differential 

 behaviour which require the most careful comparison. 



(A) First, there is the differential behaviour at telophase and 

 prophase of mitosis. Certain parts of most of the chromosomes, 

 usually proximal or distal, stain more deeply than the rest with 

 aceto-carmine but not with Feulgen's reagent (Heitz, 1935). 

 When, as sometimes happens, these are all proximal regions, they 

 may remain condensed during the resting stage and reappear as the 

 prochromosomes referred to earlier (Heitz, 1932, cf. Ch. H). The 

 differential material or " heterochromatin " takes up a large part 

 of the sex chromosomes in mosses and was earlier thought to be 

 concerned with sex (Heitz, 1928). By contrast with ordinary 



