. 56 REPRODUCTION AND LIFE HISTORY. 
(¢) Ordinary conjugation, in which two similar cells unite, with 
fusion of their nuclei, observed in Sporozoa, Heliozoa, Flagel- 
lates, and Rhizopods. In ciliated Infusorians, the conjugation 
may be merely a temporary union, during which nuclear elements 
are interchanged, 
(d) Dimorphie conjugation, in which two cells different from one 
another fuse into one, a process well illustrated in Vorticella 
and related Infusorians, where a small, active, free-swimming 
(we may say, male) cell unites with a fixed individual of normal 
size, which may fairly be called female (see Fig. 42 and Fig. 47). 
(e) Fertilisation, in which a spermatozoon liberated from a Metazoon 
unites intimately with an ovum, usually liberated from another 
individual of the same species. 
Divergent modes of sexual reproduction.—(2) Herm- 
aphroditism is the combination of male and female sexual 
functions in varying degrees within one organism. It may 
be demonstrable in early life only, and disappear as male- 
ness or femaleness predominates in the adult. It may 
occur as a casualty or as a reversion; or it may be normal 
in the adult, e.g. in some Sponges and Ccelentera, in many 
“worms,” such as earthworm and leech, in barnacles and 
acorn-shells, in one species of oyster, in the snail, and in 
many other Bivalves and Gastropods, in Tunicates and in 
the hag-fish. In most cases, though these animals are 
bisexual, they produce ova at one period and spermatozoa 
at another (dichogamy). It rarely occurs (e.g. in some 
parasitic worms) that the ova of a hermaphrodite are 
fertilised by the sperms of the same animal (autogamy). 
Certain facts, such as the occurrence of hermaphrodite 
organs as a transitory stage in the development of the 
embryos of many unisexual animals (e.g. frog and bird), 
suggest that hermaphroditism is a primitive condition, and 
that the unisexual condition of permanent maleness or 
femaleness is a secondary differentiation. Other facts, such 
as the hermaphroditism of many parasites, where cross- 
fertilisation would be difficult, suggest that the bisexual 
condition may have arisen as a secondary adaptation. It 
seems likely that there is both primitive and secondary 
hermaphroditism. 
(4) Parthenogenesis, as we know it, is a degenerate form 
ofsexuals reproduction, in which ova produced by female 
organism develop without being fertilised by male elements. 
It is well illustrated by Rotifers, in which fertilisation is the 
