258 CHARLES W. METZ 



SUMMARY AND CONCLUSIONS 



1. The chromosomes of about eighty species of Diptera have 

 been examined with especial reference to the phenomena of 

 chromosome pairing. The species studied range from among 

 the lowest to among the highest families in the order. In a 

 large proportion of cases the studies include somatic, spermato- 

 gonial and spermatocyte, or somatic and ovarian cells. 



2. In all of these species the chromosomes were found to be 

 uniformly associated in pairs in diploid cells. The only irregulari- 

 ties were occasionally displacements involving one or two pairs. 



3. The paired association was found to be characteristic of 

 all tissues, somatic as well as germinal. 



4. It was found to continue throughout all stages of cell divi- 

 sion from earliest prophase to latest anaphase, being most in- 

 timate in the earliest and latest stages, and least intimate in 

 metaphase. Telophases and resting nuclei were not favorable 

 for study. 



5. Association of paternal with maternal chromosomes ap- 

 parently is effected in early cleavage stages (perhaps before 

 the first cleavage), since in late cleavage stages the chromosomes 

 are definitely paired. 



6. The paired association was found to continue during all 

 stages in ontogeny, from the egg to the adult. 



7. Certain cases of multiple chromosome numbers (tetraploid 

 or higher multiples) were found in occasional cells. In these 

 cases corresponding chromosomes were associated together 

 in prophase in aggregates of four, eight, etc., instead of being 

 arranged in pairs. 



8. In many species several (in some cases nearly all) pairs 

 of chromosomes could be individually distinguished by char- 

 acteristics of size and form.^^^ These pairs, with the exception 

 of the sex-chromosomes in males, were in all cases symmetrical, 

 i.e., composed of similar members. 



1*^ Since this paper was sent to press I have found a species of Drosophila in 

 which each pair of chromosomes is very clearly differentiated from all others. 



