n LIFE-CYCLE OF CLADOCERA 49 



__ is invariably a parthenogenetic female : the charac". - the 

 succeeding generations differ in different c ises. 



In a few forms, of which Moin-i is the best known, the 

 parthenogenetic female, produced from a winte:- __ jive 



rise to males, to sexual females, and to parthenogenetic fern 

 so that the cycle of forms which intervene between one winter- 



__ and the next is short. A sexual female produces one 'T 

 two winter- eggs, and if these are fertilised they are end - 

 in an ephippium and cast off: if. however, the eggs when ripe 

 ar- not fertilised, they atrophy, and the female produce- partheno- 

 geueti* ggSj being thenceforward incapable of form:: _ - ;ual 

 " winter " eggs. An accidental absence of males may thus L< 

 to the < x-ciirrence of parthenogenesis in the whole of the second 

 generation. The regular production of sexual individuals in the 

 second generation from the winter-egg appears to depend on a 

 variety of circumstances not yet nndersl d. Mr. G. H. 



liosvenor tells me that M from the neighbourhood of 

 Oxford may give rise to several successive general as 

 parthenogenetic individuals, when grown in small aquaria. 



In the greater number of I'aphuiidae. the partheuo, 

 female, produced from a winter - egg, gives ri.-e only 

 parthenogenetic forms, and it is not until after half a dozen 

 parthenogenetie generations have been produced that a : - ual 

 forms appear, mixed with the others S - xual forms are fairly 

 common in April or May in this country: they pr-'duce 

 " winter " egg? and then die. the generations which succeed them 

 through the summer being entirely partheuogenetic. In 

 autumn sexual individuals are again ] giving rise 1 



plentiful crop of winter-eggs, but many parthenogenetic fern 

 ar-- still found, and some of these appear t live and to 

 pr.'diu-e through the winter. 



In >'./-/. in the Polvphemidae and Lept<xl .ridae. and i: 

 of the I.ynceidae. sexual individuals are produced <>i;l; in 



every year, while in a few forms which inhabit _ 



- . ;al condition occurs so rarely that i' - - :11 unknown. 



VY.-ismaiin 1 lias pointed out that the sexual : - with t: 

 property of producing gga which can endui 

 most frequently iu species such as Mbina, which inliabit small 



} Is liable to be dried up at frequent inter ;he 



1 Z 



VOL. IV E 



