Chromosome Elimination 489 



early cleavage stages since it is eliminated from the somatic cells 

 during an early stage of somatic development. One or two addi- 

 tional limited chromosomes may apparently also be present with- 

 out any resulting serious effect, but three is the largest number 

 ever observed in one individual. These chromosomes appear to 

 break up readily into fragments without any harmful effect. 

 They also remain heteropycnotic in the primary spermatocyte, 

 which would suggest that they may represent a true sex chromo- 

 some that had lost its function since, as we have just seen, sex 

 chromosomes in other organisms may remain heteropycnotic. 



One of the peculiar features of the chromosomes in Sciara is 

 chromosome elimination. In *S. coprophila (Fig. 141) the zygote 

 may receive two rod-shaped and one V-shaped autosome from 

 the egg and similar chromosomes from the sperm. The egg may 

 also contribute one limited chromosome and one X chromosome 

 whereas the sperm adds two limited chromosomes and two X 

 chromosomes. The germ line and soma become set apart from 

 one another during the first few cleavage divisions of the egg. 

 At the fifth or sixth cleavage division the mitoses of the somatic 

 line show certain peculiarities not found in the usual mitoses. 

 During either of these divisions all the chromosomes behave 

 normally except the limited chromosomes. The daughter halves 

 of these chromosomes separate normally but do not pass along 

 the spindle to the opposite poles. They lag on or near the 

 equator and do not become included in the daughter nuclei, but 

 they remain by themselves in the cytoplasm where they go 

 through the usual mitotic changes for one or two divisions. 

 Eventually they degenerate. At the seventh or eighth cleavage 

 division a somewhat similar elimination of the X chromosomes 

 occurs. One X chromosome becomes eliminated from eggs des- 

 tined to produce females, and there is reason to believe that this 

 is one of the two X chromosomes that entered from the sperm. 

 However, two X chromosomes are eliminated from eggs destined 

 to produce males, and it appears that both are X chromosomes 

 that were contributed by the male parent. 



In addition to this unusual elimination of chromosomes from 

 somatic cells, there is also apparently an elimination from the 

 germ cells, a process which, until recently, has not been so well 

 understood. This elimination appears to occur at a later stage 

 than elimination from the somatic line and in all eggs apparently 



