DEPARTMENT OF EXPERIMENTAL EVOLUTION. 129 



Rhizopus toxin, upon which experiments are still being made with the 

 assistance of the U. S. Department of Agriculture. 



Dr. Harris and Mr. Lawrence spent some time in Jamaica studying 

 the osmotic pressure of vegetable saps. Mr. Metz spent the winter 

 months (January 15 to April 15) in Cuba and a few days in Jamaica, 

 collecting and attempting to hybridize the Drosophila and allied flies 

 there. He brought back numerous species to breed at the laboratory. 

 Also pinned material, eggs, and notes on eggs, larvae, pupae, breeding 

 habits, etc., were procured. Cuba proved to be unusually rich in 

 Drosophila. Dr. A. M. Banta has spent the usual amount of time in 

 exploring caves for animals to replenish our stock. 



REPORTS ON INVESTIGATIONS IN PROGRESS. 



The topics upon which work has been done in tliis Department during 

 the past year may be classified as follows: (1) the germ-plasm and its 

 modification; (2) the control of sex; (3) the inheritance of germinal 

 peculiarities, especially those relating to sex ; (4) the processes of form- 

 ing species in nature and artificially; (5) general physiological topics. 



THE GERM-PLASM AND ITS MODIFICATION. 



More and more clear does it become that evolutionary changes are 

 initiated in the germ-plasm — the chromosomes and adnexa. It there- 

 fore becomes of the first importance (a) to study the structure of the 

 germ-plasm and the morphological differences in the chromosomes asso- 

 ciated with differences in the soma; (6) to try to modify directly the 

 composition and product of the germ-plasm. 



MORPHOLOGICAL EVOLUTION OF THE GERM-PLASM. 



This phase of our work is being carried out by Mr. Metz, who is using 

 in his studies the genus Drosophila and the closely related genera of 

 Chy7?iomyza, Scaptomyza, and Cladochceta. Over 30 related species 

 were thus examined cytologically. In these species 10 or 11 different 

 types of chromosome complexes have been revealed; and, most impor- 

 tant of all, these types are all closely related morphologically and can 

 be derived, hypothetically, from an unspecialized central type by simple 

 modifications in two or three directions. Thus, Mr. Metz has greatly 

 extended the results of his studies that were briefly described in the 

 Year Book for 1914 (p. 129). The analysis of these various t>T)es of 

 chromosome groups and the task of relating them to one another has 

 been rendered more difficult because of the interesting and complicated 

 relations of the pair of sex-chromosomes. In some species the sex- 

 chromosomes appear to be alike in both sexes {i. e., X and Y are equal 

 in size) , while in other species that have the same tjrpe of chromosome 

 group, X and Y are obviously dimorphic. This study appears to be 

 the first in which the chromosome group has been analyzed in a large 



