Genie and Non-genic Parts of the Chromosome 63 



different, are found in a few Diptera. The most extreme and com- 

 plicated type is that of the manure fly Sciara. Without entering into 

 all details of the chromosomal cycle (see fig. 7), the following facts 

 have been described (Metz, Dubois, and others; see Metz, 1938). All 

 germ-track cells have three pairs of autosomes, one (male) or two 

 (female) X-chromosomes and two very large "sex-limited" chromo- 

 somes, actually two completely heterochromatic chromosomes, com- 

 parable to the chromosomal ends in Ascaris, but provided with 

 centromeres and thus dividing normally in the germ-track cells. All 

 somatic cells are devoid of these two heterochromatic elements. The 

 heterochromatic element (not present in all species) shows also 

 heteropyknosis. There are some strange complications in chromosome 

 numbers which are rectified during development (shown in the 

 diagram, but not of interest in our present discussion). The main 

 point is that in development these large heterochromatic chromosomes 

 are removed from the nuclei of the somatic cells in a manner strictly 

 parallel to the situation in Ascaris. One suspects that something is 

 WTong with the centromere of these chromosomes. 



Another variant is found in the gallflies (Kahle and many suc- 

 cessors; see White's book, 1945, and White, 1950). Here again the 

 germ-track cells have diflFerent chromosome complements from the 

 somatic cells. In the same way as before, a diminution takes place in 

 early development, when germ cells and somatic cells are segregated, 

 but here a clear cytological distinction between heterochromatic and 

 other chromosomes has not been possible. The microscopic picture 

 shows many (64 in one case) chromosomes in the fertilized egg and 

 in the cells of the germ track. When diminution takes place in the 

 cleavage cells of the embryo, the majority of these chromosomes (in 

 one case, all but 10) are removed in one or two divisions by re- 

 maining in the equator of the spindle and being excluded from the 

 daughter nuclei. Again something must have happened in the centro- 

 meric apparatus under the influence of the segregated cytoplasm of 

 the potential somatic cells, something which does not happen in the 

 cytoplasm of the germ-track cells (known to be cytoplasmically dif- 

 ferent; this is true also of Ascaris, as Boveri has already emphasized ) . 

 The facts are similar to those discussed for Infusoria: a cytoplasmic 

 gradient or field controls happenings in the self-duplicating organelles 

 of division, the kinetosomes there, the centromeres here. So far, no 

 proof of the heterochromatic nature of the diminished chromosomes 

 has been found ( heteropyknosis ) , but I cannot see how we can escape 

 the conclusion that we are dealing with the same thing. Actually 



