224 REPORTS OF INVESTIGATIONS AND PROJECTS. 



years all hybrids reared have been completely sterile, a female hybrid has 

 this year been obtained which is fertile with the wild male. The hybrids 

 show blending inheritance as regards bodily proportions and apparent Men- 

 delian inheritance as regards color characters, but a cross of the hybrid with 

 the guinea-pig is required fully to establish the occurrence of Mendelian 

 segregation. 



The polydactylous race of guinea-pigs described in Publication No. 49, 

 which was produced from a single abnormal individual, remains entirely 

 stable. 



Three student-assistants have rendered valuable aid in the prosecution of 

 the experiments, viz, W. G. Vinal, E. L. Walker, and W. E. Wing. 



On the material side the experiments have been more successful than in 

 any previous year. The number of guinea-pigs reared in this series of ex- 

 periments now amounts to over 11,000; the number of rabbits to over 1,500, 

 and that of rats to over 4,000. Epidemics occurring in cold weather are 

 still the chief obstacle to the guinea-pig experiments. The crowded con- 

 dition of our animal-room is undoubtedly in part responsible for this, a dififi- 

 culty which is soon to be remedied, through the generous cooperation of the 

 Harvard Corporation. 



Dr. Mark reports that as material for the study of spermatogenesis and 

 ovogenesis in the hybrids between normal and rumpless fowls has not been 

 secured in sufficient quantities during the year, attention has been given 

 to the two related lines of work referred to in his report a year ago. 



(i) In studying spermatogenesis in hybrid guinea-pigs it seemed de- 

 sirable to make comparisons with cases of abnormal spermatogenesis in 

 non-hybrid animals, due to diseased conditions of the animal. Consequently 

 the work of Mr. Gulick was extended to cover two such cases — one the male 

 of Cavia cobaya; the other that of C. aperea. In both instances the dis- 

 eased condition of the animal led to conditions of degeneration in the sexual 

 products, which were in most particulars not to be distinguished from those 

 seen in the apparently healthy hybrids resulting from the crossing of a 

 male C. aperea with females of the domestic species (C cohaya). It further 

 appears that there are probably two periods in spermatogenesis at which the 

 evidences of abnormality (degeneration) are more conspicuous than at 

 others, viz, during synapses and mitoses, especially those of the primary 

 spermatocytes. The results of this study are nearly ready for presentation, 

 but the observations ought to be extended to cover a greater number of 

 cases before a final paper is published. 



(2) In collaboration with Mr. Manton Copeland, work on spermatogenesis 

 in Hymenoptera has been continued according to the plan mentioned in the 

 last report. The conditions in the mud wasp (Vespa maculata Linnaeus) have 

 been studied, and a brief account of the results has been published. 



