THE VITAL PROPERTIES OF THE CELL 297 



This may be concluded from the fact, that, in certain species, the 

 generation-cycle assumes a different appearance, according to the 

 conditions of life under which the animals are living. 



The inhabitants of small shallow pools, which readily dry up, 

 produce only one, or at most a few generations of females, which 

 multiply asexually; after this ova requiring fertilisation are 

 produced, so that in the course of a year several generation-cycles 

 (consisting of unimpregnated females and sexual animals) suc- 

 ceed each other. The inhabitants of lakes and seas, on the other 

 hand, produce a long series of unimpregnated females before de- 

 positing ova, which require fertilisation ; this occurs towards the 

 end of the warm season. A generation-cycle, therefore, in this 

 case occupies a whole year (polycyclical and monocyclical species 

 of Weismann). 



Weismann (VII. 39), who investigated the whole subject most 

 thoroughly, remarks : " That asexual and bi-sexual generations 

 alternate with one another in various ways in Daphnida3, and 

 that the mode of their alternation stands in a remarkable relation 

 to their environment. According to whether the causes of de- 

 struction (cold, desiccation, etc.) visit a colony several times 

 during the year, or once, or not at all, we find Daphnoids which 

 exhibit several cycles within a year, others which have only one 

 cycle, and finally there are species which do not exhibit any 

 generation-cycle at all; hence we can distinguish between poly- 

 cyclical, monocyclical and acyclical forms." 



In many species, which are exposed to frequently changing con- 

 ditions, we notice, that some of the ova, which are formed in the 

 ovary, develop into summer eggs, whilst others have a tendency 

 to become winter eggs. According to Weismann, "a war, as it 

 were, goes on to a certain extent in the body of a female between 

 the tendencies to form these two kinds of eggs." 



In Daphnia pulex, the germ of a winter egg may often be re- 

 cognised amongst several summer eggs in the ovary ; this grows 

 for a few days, even beginning to accumulate the finely granular, 

 characteristic yolk; but then it is arrested in its development, 

 becomes gradually dissolved, and finally completely disappears. 

 If winter eggs have been developed, but owing to the absence of 

 the males, have not become fertilised, they disintegrate after a 

 time, and summer eggs are again formed. 



How can it be explained, then, that, amongst eggs which have 

 been developed one after another in the same ovary, some 



