MODES OF SEXUAL REPRODUCTION. 57 
exception (in some genera males have never been found), 
by many small Crustaceans whose males are absent for a 
season; by Aphides, from among which males may be 
absent for the summer (or in artificial conditions for several 
years) without affecting the rapid succession of female 
generations ; by the production of drones in the bee-hive 
from eggs which are never fertilised. 
(c) Alternation of generations. A fixed asexual hydroid 
or zoophyte often buds off and liberates sexual medusoids 
or swimming-bells, whose 
fertilised ova develop in- ee, ee 
to embryos which become I . 
fixed and grow into hydroids 4 tor or 
(Fig. 71, p. 150). This is 
the simplest illustration of 
alternation of generations, Grow 
which may be defined as .p © 
the alternate occurrence in a ar" 
one life-cycle of two (or more) 
different forms differently ¥ic. 26.—Diagrammatic expression 
produced (Fig. 26): of alternation of generations. 
The liver-fluke (Distomum 1. Hydromeduse, 
ov. Fertilised ovum gives rise to an 
hepaticum) of the sheep asexual form 4, which, by bud- 
produces eggs which, when fae Gane Be one Cee 
fertilised, grow into embryos. medusz, A is represented by 
Within the latter, certain eo ee 
cells (which might be called 7 pertlioet ovum gives rise to 
spores) grow into numerous paae e see Nad i, from 
Dp special spore-like cells pro- 
ee Ee = ae a eae the sexual 
orm. ithin these e fluke (5). 
same process is repeated, 
and finally the larvee thus produced grow (in certain con- 
ditions) into sexual flukes (Fig. 98, p. 189). In this case, 
reproduction by special cells, like undifferentiated precocious 
ova, alternates with reproduction by ordinary fertilised egg- 
cells. So, too, the vegetative sexless “fern-plant” gives rise 
to special spore cells, which develop into an inconspicuous 
bisexual “ prothallus,” from the fertilised egg-cell of which 
a “fern-plant” springs. 
Various kinds of alternation are seen in the life-cycle of 
the fresh-water sponge, in the stages of the jelly-fish Auzelza, 
