106 CARNEGIE INSTITUTION OF WASHINGTON. 



In making a test of the above hypothesis, Dr. Riddle obtained a relatively- 

 large series of eggs of varying stages of prematurity and subjected them to 

 various temperatures during variable periods. His results strongly indicate 

 that twins and double monsters are not thus produced in pigeons. From 200 

 adequate tests there resulted 192 normal embryos, 5 with some abnormality 

 probable, 3 possibly though not probably of double nature, and none plainly 

 twins or double monsters. When these data are reviewed in the light of other 

 limiting circumstances previously reported, the result is, it is believed, a 

 conclusive demonstration that the particular twins on which our studies are 

 based were not the result of premature laying with consequent modification 

 of the gastrulation process. The evidence which our cases of twins afford 

 for the relation of high yolk storage to femaleness remains, therefore, quite 

 unimpaired. 



Control of the Sex-Ratio. 



Attention has been called in earlier Year Books to the attempts made by 

 Dr. A. M. Banta to control the sex-ratio in Cladocera. In certain lines in 

 this group males have never been observed; in others, males occasionally 

 appear. The conditions that determine their appearance have never been 

 discovered. With the collaboration of Mr. L. A. Brown, of the University 

 of Pittsburgh, important progress has been made during the past year, first, 

 in determining the critical period during which the sex of the offspring of 

 Moina macrocopa is subject to experimental control; second, in the elimina- 

 tion of a number of environmental factors as primary agents in sex control; 

 third, in securing, by experiment, suggestions as to the specific environmental 

 factors involved. 



To find the stage in the developing egg at which sex is determined, 32 experi- 

 ments were made (involving a record of 6,895 young) in which females were 

 removed from the crowded bottles at various instars. No males appear 

 among the young of mothers relieved from crowding prior to the late third 

 instar. But if the transfer was delayed until the beginning of the fourth 

 instar (when the eggs are in the brood pouch) the normal proportion of male 

 offspring appeared. It is concluded, accordingly, that in this species sex is 

 determined immediately before the eggs leave the ovary; and this is known, 

 in some species of Cladocera, to be the time of maturation of the partheno- 

 genetic egg. 



As for the second and third points, it was known from our earlier experi- 

 ments that in 95 per cent of the cases males were produced in bottles con- 

 taining 10 or more mothers, if the bottles were left undisturbed, while bottles 

 containmg only 1 mother never yielded a first brood of male young. 



Two groups of experiments were conducted. In the first group aeration of 

 the culture water was accomplished (1) by bubbling air through the test 

 bottles, (2) by bubbling oxygen through the bottles, (3) by shaking the 

 bottles with air, and (4) by shaking the bottles with oxygen. Thirty experi- 

 ments of this type gave 4,364 control young, of which 38.7 per cent were males, 

 while in the tests there were 3,870 young, of which 24.9 per cent were males. 

 Thus the aeration apparently reduced the percentage of males by one-third. 



In the second group the mothers themselves were treated by aerating them 

 on a slide in a thin film of water. Of 2,122 young of the controls, 55.9 per 



