Genie and Non-genlc Parts of the Chromosome 69 



the second, a role of heterochromatin in cell division, but runs in the 

 opposite direction from the second h>-pothesis. It would permit the 

 inclusion of the Dyticus case, because the egg cell is one which for a 

 long time grows without division. It would also permit an explanation 

 why insects and especially cecidomjids are so conspicuous: their 

 development is an extraordinarily fast one, meaning ver\' quick 

 division of the somatic cells ( see, however, \Miite, 1950 • . But here as 

 before remains the question why only the few examples are known. 

 It is hardly imaginable that such a comphcated and conspicuous 

 cytological feature is meaningless. It is also impossible that an im- 

 portant process takes place, say, in one species of Sciara and not in 

 another. We are almost forced to assume that comparable processes 

 are ubiquitous but only rarely organized on the level of microscopic 

 visibiht)-. At present I prefer this explanation (no. 3) to oui former 

 one (no. 2). Both have their good points and their difficulties. 



4. A variant of the interpretation just discussed was proposed 

 earHer by Painter (1945). He starts from the fact that RNA plays a 

 role in the functions of the c>toplasm as a source of protein s>"nthesis 

 and of s>Tithesis of DXA for the nuclei. Therefore, quickly di\iding 

 cells, the somatic cells in development, have a greater need for RXA 

 which is being satisfied by the eliminated heterochromatin; the latter, 

 as genetically inert is not needed in the nuclei. This point of \"iew 

 does not emphasize presence or absence of heterochromatin after 

 diminution, but the diminished material itself as a source of RNA for 

 use in cell propagation. The presence of supernumeraries in plant root 

 cells is considered to have the same meaning. In favor of such an 

 idea is a strange phenomenon (discussed I 2 B a), the sloughing oflF 

 of chromosomal material in the meiotic di\isions of lepidopteran eggs, 

 discovered by Seiler ( 1914 ). It is known now that this material is 

 RXA. But there are difficulties for a comparison with real chromatin 

 diminution. There is no heterochromatin involved; the time of the 

 happening is too late for a role in egg organization and too early for 

 a role in development; further, the crosslike mass of sloughed-o£F RXA 

 can be seen unchanged for a considerable time and does not seem 

 to contribute an^liiing to development which is also true of the 

 sloughed-off heterochromatin in Ascaris. 



5. There is still another interpretation of heterochromatin which 

 does not consider all the remarkable facts discussed above but deals 

 only with the general aspects of heterochromatin distribution in the 

 chromosomes and its relation to chromosome structure. We have dis- 

 cussed (in I 2 B a) Serra's \iews of the intimate structure of the chro- 



