102 CARNEGIE INSTITUTION OF WASHINGTON. 



excretory products or related substances — ammonium hydrate, urea, urine, 

 chicken manure, and treatment with alcohol, sulphuric acid, and sodium 

 hydrate — have resulted in reduced male production. On the other hand, 

 animals treated with carbon dioxide, or uric acid, or reared under pressure, 

 have produced increased percentages of males." 



Control of Production and Hatching of Sexual Eggs. 



Previous knowledge of production of sexual eggs in Cladocera (in which 

 only parthenogenetic eggs are ordinarily produced) has been extended. 

 Control measures are somewhat difficult, but means of an entirely practicable 

 sort have been developed for Moina macrocopa and Daphnia longispina and 

 one of our types of Daphnia pulex. There is reason to think that these 

 measures may be readily modified so as to apply to most, if not all, the other 

 Cladocera under cultivation. Hatching the fertilized sexual eggs is more 

 difficult than their production, but those of two of our forms may now readily 

 be hatched and some of those of Daphnia longispina have been successfully 

 hatched into viable offspring. The occurrence of sex intergrades in two of 

 our laboratory stocks and of two other clear-cut mutations in Daphnia longis- 

 pina makes hybrid matings a matter of much interest. Our attempts at 

 hybridization have so far been unsuccessful, but will be continued. 



Cytological Work on Cladocera. 



The occurrence of sex intergrades in two species of our Cladocera stocks, 

 and the ability to control through environmental conditions the sex of Cla- 

 docera, make the chromosome situation in these animals a matter of much 

 theoretical interest. Cytologically, Cladocera are very difficult, and the 

 services of a good cytologist seemed essential in attacking this problem. Dr. 

 Ezra Allen, of Ursinus College, whose improvements in cytological technique 

 have made accurate studies in mammalian cytology possible, spent two 

 months with us this summer and made a good start upon the study of the 

 cytology of two species of Cladocera. He reports as follows: 



"Fixation was made in Fleming's fluid, 10 per cent formahn, Dantchakov's, 

 and B-15 (Allen's modification of Bouin). Fleming proved of no value for 

 the large eggs; its effect upon the chromosomes in the younger tissues has 

 not yet been studied. The fixation of the testis proved quite satisfactory 

 in B-15. 



"As to stages studied, since the chief interest in this form lies in the sex 

 differentiation by the parthenogenetic female, the period of, and immediately 

 following, egg-laying was studied at first in hope of determining the matura- 

 tion phenomena. The large quantity in the yolk of coloring matter that 

 takes both hematoxylin and safranin made this an undesirable and difficult 

 stage to do the first work upon. Consequently embryonic and post-embry- 

 onic stages were studied, in females only at first. These consisted of em- 

 bryos fixed at 13.5, 16, and 20 hours after egg-laying, and of young about 

 ready to be released from the brood-pouch. The young were fixed at about 

 2, 8, 16, 18, 24 to 26, and 36 hours after release from the brood-pouch. Young 

 males were fixed at 15, 18, 24, 12 to 36, 30, 24, 48, 60, and 72 hours after 

 release. 



"The results were that no dividing oogonia were found. Cell division was 

 active in the testes of the 24 and 60 hour stages. The older males have not 

 yet been studied. The cells are very small and not favorable to study. 

 Apparently the haploid number is in the neighborhood of 8 chromosomes, 



