EXPERIMENTAL EVOLUTION. 



REPORTS ON INVESTIGATIONS IN PROGRESS. 



THE GERM-PLASM AND ITS MODIFICATION. 



COMPARATIVE STUDY OF THE CHROMOSOME GROUPS IN DIPTERA. 



Dr. C. W. Metz, in association with Dr. J. F. Nonidez and Mrs. 

 Rebecca C. Lancefield, has continued his studies on the chromosomes 

 of Drosopkila and other Diptera. Dr. Metz reports that, owing to the 

 peculiarities of chromosome behavior (especially paired association) 

 in the flies and the possible bearing of these peculiarities on genetical 

 phenomena, it has seemed desirable to make a detailed study of the 

 maturation processes in this group. Since no one species offers the 

 best technical conditions, and since the cytological processes seem to 

 show differences, there is being made a comparative study of numerous 

 species scattered through the order. A study of spermatogenesis in 

 two species of robber flies (Asilus) by Metz and Nonidez has been com- 

 pleted, a similar study of a stratiomyid fly (Ptedicus trivitattus) is nearly 

 finished, and studies in other families are partially completed ; studies 

 on oogenesis in three families of Diptera are under way. 



The studies in Asilus yielded a result of great theoretical importance. 

 In the zygote, as is well known, each kind of chromosome is paired, one 

 of each pair coming from the egg and one from the sperm. In a cell- 

 generation before the ripe gametes are formed the "homologous" mem- 

 bers of the pairs come together in what is called "synapsis." In the 

 following cell-division the two members of each synaptic pair separate, 

 one going to each daughter-cell, so that the ripe gamete contains only 

 one of each homologous pair. Now Metz has found that in Asilus the 

 homologous chromosomes remain closely associated throughout the 

 entire growth-period of the first spermatocyte, with a consequent modi- 

 fication of the synaptic processes, due to the ehmination of the lepto- 

 tene and zygotene stages. True synapsis occurs in the telophase of 

 the last spermatogonial division. 



COMPARATIVE GENETICAL STUDIES ON DROSOPHILA. 



The studies on oogenesis, Dr. Metz reports, although incomplete, 

 suggest that in the female the processes may be different from those in 

 the male, and that the difference may be responsible for the difference 

 in genetical behavior in the two sexes (crossing-over in the female 

 but not in the male) of Drosophila. This study is being extended in 

 the hope that the question may be settled by a combination of cyto- 

 logical and genetical work on favorable material, as follows: 



"In conjunction with the cytological studies on Drosophila, considerable 

 genetical work has been carried on for the purpose of analyzing the genetical 

 constitution of the chromosomes in different species of Drosophila and ascer- 



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