DEPARTMENT OF GENETICS. 107 



cent were males, and of 2,706 young of the tests only 19.5 per cent were males. 

 Thus the treatment of the mothers apparently reduced the number of their 

 male young by nearly two-thirds. 



Experiments were planned to test whether or not this result was due to the 

 reduction of carbon dioxide. Uncrowded mothers were treated with carbon 

 dioxide. If the crowding effect was due to carbon dioxide, this treatment 

 should give males. Twenty-one experiments were attempted in which the 

 amouut of carbon dioxide given ranged from 20 bubbles to a lethal dose. 

 All of the 3,285 young produced were female. Hence it would seem that 

 male prodnction is not determined by an excess of carbon dioxide in the cul- 

 ture water. 



The next expedient tried was the removal of oxygen from the uncrowded 

 bottles by streaming nitrogen through the culture water. The 3,969 young 

 resulting, except for a few chance cases, were females. 



Six experiments have been completed, in which nitrogen was bubbled into 

 crowded bottles. These experiments included 206 mothers and 1,240 young. 

 The crowded controls, as usual, gave many males (40.9 per cent), and the 

 treated bottles gave 27.3 per cent males, a difference of one-third. 



From the foregoing brief summaries of Dr. Banta's experiments it will be 

 seen that, while aeration decreases the percentage of males in crowded bottles, 

 an increase in carbon dioxide or a decrease in the amount of oxygen in un- 

 crowded bottles fails to cause the production of males. It will also be noticed 

 that nitrogen apparently produces somewhat the same result when bubbled 

 through a crowded bottle as is accomplished by the aeration. 



Control of Production of Sexual Eggs. 



Associated with the problem of control of the sex ratio in Daphnia is that 

 of the experimental control of the production of that peculiar type of egg 

 which will not develop unless fertilized, called ephippial egg. This control 

 has been secured, in Dr. Banta's experiments, with two of the laboratory 

 forms and partial control has been secured for a third form. 



"With one of the (?) Daphnia pulex types, the '984 type,' the simple expedient 

 of crowding the young from birth until they are sexually mature determines 

 the production (not of males as in all other forms similarly treated but) of 

 'ephippia' bearing the sexual eggs. With Moina macrocopa ephippial eggs 

 are produced by females crowded in strained spring water from soon after 

 birth until sexual maturity. This species may also usually be caused to 

 produce ephippial eggs by crowding the young females in old, somewhat 

 depleted food. It would seem that paucity of food and the accumulation of 

 waste products may possibly both be involved in this control measure. 



11 Daphnia longispina is likewise somewhat amenable to the last-mentioned 

 treatment as a means of ephippial production. 



" The '984 type' has not been known to produce males, although it has a long 

 laboratory history and has been designedly subjected to rather varied condi- 

 tions at different times. It is believed that males do not occur in this form. 

 Of further interest is the anomalous discovery that in this form 'sexual' 

 eggs develop asexually. 



"To the previous evidence bearing on the non-occurrence of the supposed 

 internal sexual cycle, (1) the long-continued parthenogenetic reproduction, 

 (2) the undiminished vigor under continued parthenogenesis, and (3) the fact 



