Life-cycle of the Lower Crtistaceans 343 



composed of 70 broods. There were about 500 individuals 

 produced. All were parthenogenetic, and no males and no 

 sexual females appeared. In another similar series a few males 

 and sexual females developed, but the tendency seemed to be to 

 produce purely parthenogenetic forms. When kept at room 

 temperature, 16° C, an isolated parthenogenetic female pro- 

 duced from February 6 to April 15 two generations, and 13 

 broods in all, composed of about 100 individuals. The first 

 brood consisted of females and 5 males. The sixth brood was 

 composed of parthenogenetic females which brought forth males, 

 etc. Another culture under the s^me conditions gave 30 broods 

 of 250 individuals, Again there was observed a transition from 

 females to males through an intermediate mixed brood. In both 

 cultures the females of the first generation produced the shell 

 for the winter egg — the so-called ephippium. This was then 

 thrown off because the egg was not fertilized. The next brood 

 of the same individuals consisted only of parthenogenetic fe- 

 males, but the next brood after this was made up either entirely 

 of males or else another ephippium was formed. When a 

 female produces the ephippium with the winter egg in it, the 

 sexual condition is present, and therefore it will be observed 

 that the same individual may function at one time as a partheno- 

 genetic and at another time as a sexual female. In the case last 

 mentioned where another ephippium is produced this may be 

 thrown off and the following brood will be again males or an- 

 other ephippium develops. When kept in the cold, 6° C, there 

 is a still stronger tendency to produce sexual forms. This is 

 shown in the two following experiments. All the offspring of 

 one female were taken from the warm culture and separated 

 into two groups. One set was kept at 24° and the other at 8°. 

 The results for the two cases are shown below: — 



