Genie and Non-genlc Parts of the Chromosome 61 



ations immediately before the meiotic prophase (Lepidoptera: Seiler, 

 1914; Goldschmidt, 1923Z?; some plants: Thomas, and Revell, 1946). 

 One interpretation is that this is a heterochromatic association, mean- 

 ing that heterochromatic sections of chromosomes tend to unite under 

 some conditions in the nucleus. This would, then, involve the same 

 type of unspecific attraction between heterochromatic sections which 

 sometimes produces chromocenters. Whether this interpretation ap- 

 plies to Ascaris or not is diflBcult to decide. Actually, the non-hetero- 

 chromatic individual chromosomes assembled into the collective 

 chromosome contain hardly any chromatin (Lin, 1954). The fact 

 remains that these collective chromosomes keep together only in the 

 germ cells but break apart in the somatic cells. Assuming that the 

 collective chromosome is based upon heterochromatic association, 

 the breaking up might be caused by removal of the last interchromo- 

 somal vestiges of heterochromatin, which, however, are not visible. 



These long collective chromosomes have heavy, deeply staining 

 distal ends which were proved (Goldschmidt and Lin, 1947; details 

 in Lin, 1954) to be heterochromatic (fig. 6). In the nuclei of the 

 meiotic prophase they unite into an unusually large chromocenter, 

 and in diplotene this breaks up so that the chromosome ends appear 

 as separate chromocentric blocks (Lin, 1954). Boveri showed that 

 these chromosome ends, which we must consider now as the hetero- 

 chromatic partners of the individual chromosomes within the collec- 

 tive chromosome, are broken off in the mitosis of prospective somatic 

 cells and are dissolved in the cytoplasm, while the collective chro- 

 mosome breaks up into its constituents. We may say now that these 

 heterochromatic ends do not have a centromere of their own, as is also 

 visible in the division configuration of the collective chromosome, and 

 therefore are removed from mitotic distribution once the collective 

 chromosome breaks up into the many somatic chromosomes. Before 

 that time and in all germ-track cells in which no diminution takes 

 place, the heterochromatic ends show the heteropyknosis and allocycly 

 (and staining reactions) of heterochromatin. The lack of a centro- 

 mere puts these individual partners of the somatic chromosomes 

 within the collective chromosomes in the same category as super- 

 numeraries, but with much more regularized, orderly behavior be- 

 cause of their inclusion in the collective chromosome. The final effect 

 of this heterochromatic behavior in Ascaris is the conservation of all 

 heterochromatin in the germ-track cells and its removal from all 

 somatic cells. 



Conditions, in the end identical with these though in detail very 



