108 



CARNEGIE INSTITUTION OF WASHINGTON. 



"The occurrence of males is more general in the culture-water now being 

 used than in that formerly used. 



"Since the epidemic of males in May 1917 we have carefully examined our 

 discarded bottles of Cladocera for males. In all this stock males occurred 

 but three times before May 1917. The large number of males in the indoor 

 cultures came when the outdoor pond from which the culture-water was 

 obtained also contained vast numbers of males among the wild stock, as is 

 noted above. 



"In the several species and in the groups within the various species hav- 

 ing various periods of laboratory history, the occurrence of males has been 

 remarkably consistent. In most of the groups some males have been found 

 in every month from May 1917 to June 1918. But the point of interest lies in 

 the relative frequency of the males in the different groups. At a time when 

 males are relatively abundant within one species or a group, within a species 

 they are relatively frequent in all the other groups. When males are very 

 scarce in one group they are also scarce in the other groups. 



"Curves showing the incidence of males for the various species and groups of 

 Cladocera in the laboratory are remarkably consistent in form. Following the 

 frequency of males in May 1917, represented by a relatively high part in the 

 curve, in July and August there was a very low place in the curve, males occur- 

 ring infrequently; then came a high point in October and November; a small 

 drop in December; and the highest point of all in January; this was followed 

 by a moderately low point in April and May, with a marked rise again in June. 



"While these curves for the different species and groups with different 

 laboratory history are of course not identical, their degree of similarity is 

 very striking. When one recalls that this Cladocera stock consist? of seven 

 species, was obtained at different places, some remotely separated from 

 where most of it was derived, and at widely different times, this remarkable 

 close similarity of the frequency curve for male production is very striking. 



"The only factors which have been consistent throughout are the environ- 

 mental factors. The diverse stock has had the same food and has been 

 subject to the same environmental conditions, varying somewhat from month 

 to month, but for all the stock the same for any particular month. Hence it 

 seems clear that male production is subject to environmental control." 



The Cladocera stock now being used consists of the following: 



