96 CARNEGIE INSTITUTION OF WASHINGTON. 



show two chromosomes of each size, appearing otherwise identical, in the 

 pollen-grains (fig. 2) are probably tetraploids or descended from tetraploids. 

 On the other hand, if all the chromosomes in the pollen-grain are of different 

 sizes or shapes (fig. 1), the plant is not tetraploid or double diploid. 



A number of species, especially monocotyledons, have been studied in 

 this respect, such as Cypripedium pubescens, C. acaule, Narcissus, Hyacinthus, 

 Iris, Uvularia, Funkia, and Hemerocallis. Of these, the ordinary hyacinth, 

 styled diploid by de Mol, who counted the number of somatic chromo- 

 somes, shows in the pollen-grain (fig. 2) 2 pairs of V's, each with a clear 

 median constriction; 1 pair of large J's, shorter than the V's; and 1 pair of 

 short, often straight, chromosomes. The members of each pair are appar- 

 ently identical. It is to be inferred that these hyacinths are either tetraploid 

 or lately descended from tetraploids. 



EVOLUTION OF THE GERM-PLASM. 

 Drosophila. 



As pointed out in earlier reports, the genus Drosophila, with its numerous 

 species, affords a unique opportunity to study the differences in the chro- 

 mosomal complex of these species and to obtain the history of the changes 

 by which these differences have been produced. Dr. C. W. Metz and asso- 

 ciates have published (Carnegie Inst. Wash. Pub. No. 328, July 1923) a 

 report on their genetic studies on Drosophila virilis. This research, of which 

 the progress has been noted in earlier annual reports, shows that the varia- 

 tion from 3 to 6 pairs of chromosomes in the different species could perhaps 

 be explained on the hypothesis that the larger number of chromosomes has 

 been derived from the smaller by fission, or vice versa. If the 6 pairs of 

 chromosomes of Drosophila virilis have been derived from the 4 of D. melano- 

 gaster, then the hypothesis might be tested that at least 2 of these chromo- 

 somes should be strictly homologous between the two species, and accordingly 

 should have similar genes arranged in the same order. This hypothesis is 

 being tested and some evidence supports it for the sex-chromosome in the two 

 species. Here, yellow, crossveinless, singed, and forked are found in the sex- 

 chromosome maps of the two species in about the same relation to each other. 

 Evidence is also being obtained that minor changes, in the nature of rear- 

 rangements in the order of the genes in one small portion of a chromosome, 

 may occur. The accumulation of evidence on these matters involves the 

 finding of new mutant characters and the location of their genes on the 

 chromosome map — a laborious process. 



During the past year, Dr. Metz's group have found about 20 new mutant 

 characters in D. virilis, making 60 in all in this species. Also, several new 

 characters have been obtained in D. willistoni. Some of these new characters 

 are of particular interest, because they appear to duplicate characters known 

 in other species of Drosophila. Most of them have not been studied suffi- 

 ciently to warrant conclusions at this time, but in the case of others the 

 evidence warrants a report here. Dr. Metz reports in detail as follows: 



"In D. willistoni the sex-linked characters scute, yellow, vermilion, and 

 forked have already been shown to resemble characters in certain other 

 species and to correspond to these in sequence on the chromosome map. 

 Recently we have obtained the additional 'parallel' crossveinless, making the 



