DEPARTMENT OF GENETICS. 99 



CONTROL OF THE SEX-RATIO. 

 Attempts to control the sex-ratio in the little "water fleas," or Cladocera, 

 have been continued by Dr. A. M. Banta, with the efficient collaboration of 

 Mr. L. A. Brown, of the University of Pittsburgh. This group of organisms 

 seems especially well fitted to throw light on the ages-old problem of how the 

 sex of offspring is controlled, but the final solution is not yet in sight, though 

 we are obviously approaching nearer to it. 



Species Studied and Method Employed. 

 The main work has been done on Moina macrocopa. Since the beginning, 

 three years ago, more than 800 experiments have been made with this species, 

 and approximately 160,000 individual young have been sexed by the aid of 

 the microscope, more than 75,000 of them this summer. Upon another 

 species, Simocephalus exspinosus, a few additional experiments upon male pro- 

 duction have been made, with very gratifying results. As pointed out in 

 previous reports, the method of producing male offspring in these typically 

 female-producing species is to crowd the mothers in small bottles of water. 



The Critical Period of Sex Differentiation. 



As a preliminary to the determination of the period at which sex of off- 

 spring is fixed, it was necessary to study the developmental cycle of the organ- 

 ism. Development in Cladocera does not proceed uniformly, but by a series 

 of molts, each accompanied by a sudden increase in size. Between molts the 

 organism is at a standstill in its size; the standstill stage is called an instar. 

 The first brood is released at the end of the fourth instar, and a molt occurs 

 after each brood is set free. The total number of broods may be 14 or more. 

 The normal individuals of any instar are very similar in size and differ in 

 size from those of any other instar. Hence the number of the instar can 

 be determined by measuring the individual. As a result of over 1,200 meas- 

 urements of lengths of females, a table of sizes for each instar has been pre- 

 pared. Considering the increases in length between instars, it appears that 

 the percentage increase in length is almost constant during the young and 

 adolescent instars and the first two adult instars, but following this, as the 

 animals grow older, there is a marked decrease in the percentage increase 

 in size. These results were used also in studies on the relative metabolism 

 of the sexes. 



Experiments reported on last year indicated that sex is fixed in the ovarian 

 egg of Moina macrocopa during the latter half of the instar preceding its 

 expulsion from the ovary into the brood-chamber, i. e., in the latter half of the 

 third instar. Additional experiments have verified the earlier conclusions 

 and, in addition, limit the critical period during which sex of the young is 

 subject to experimental control to approximately 4 hours previous to the 

 passage of the egg into the brood-chamber. During this short and very 

 definite period, sex is subject to control; following it sex is absolutely fixed. 



Analysis of Sex-control Factors. 

 As stated in previous Year Books, the simple expedient of crowding Moina 

 macrocopa and other Cladocera species causes the production of males. Most 

 of the experiments to be reported now consisted of such crowded or semi- 

 crowded control bottles in experiments in which the tests were modified in 

 one way or another. Every experiment has a control and a test bottle and 



