II LIFE-CYCLE OF CLADOCERA 49 
egg is invariably a parthenogenetic female: the characters of the 
succeeding generations differ in different cases. 
In a few forms, of which JJoina is the best known, the 
parthenogenetic female, produced from a winter-egg, may give 
rise to males, to sexual females, and to parthenogenetic females, 
so that the cycle of forms which intervene between one winter- 
egg and the next is short. A sexual female produces one or 
two winter-eggs, and if these are fertilised they are enclosed 
in an ephippium and cast off; if, however, the eggs when ripe 
are not fertilised, they atrophy, and the female produces partheno- 
genetic eggs, being thenceforward incapable of forming sexual 
“winter” eggs. An accidental absence of males may thus lead 
to the occurrence 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 understood. Mr. G. H. 
Grosvenor tells me that JMJoina from the neighbourhood of 
Oxford may give rise to several successive generations of 
parthenogenetic individuals, when grown in small aquaria. 
In the greater number of Daphniidae, the parthenogenetic 
female, produced from a winter-egg, gives rise only to 
parthenogenetic forms, and it is not until after half a dozen 
parthenogenetic generations have been produced that a few sexual 
forms appear, mixed with the others. Such sexual forms are fairly 
common in April or May in this country; they produce 
“winter” eggs and then die, the generations which succeed them 
through the summer being entirely parthenogenetic. In late 
autumn sexual individuals are again produced, giving rise to a 
plentiful crop of winter-eggs, but many parthenogenetic females 
are still found, and some of these appear to live and to re- 
produce through the winter. 
In Sida, in the Polyphemidae and Leptodoridae, and in most 
of the Lynceidae, sexual individuals are produced only once in 
every year, while in a few forms which inhabit great lakes the 
sexual condition occurs so rarely that it is still unknown. 
Weismann ' has pointed out that the sexual forms, with their 
property of producing eggs which can endure desiccation, recur 
most frequently in species such as Moina, which inhabit small 
pools lable to be dried up at frequent intervals, while the 
1 Zeitschr. wiss. Zool. xxvil., XXxiil., 1876, 1879. 
VOL. IV E 
