56 REPRODUCTION AND LIFE HISTORY. 



(c) 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) Dimorphic 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. (a) 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 Coelentera, 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. 



() Parthenogenesis, as we know it, is a degenerate form 

 of sexual reproduction, in which ova produced by a female 

 organism develop without being fertilised by male elements. 

 It is well illustrated by Rotifers, in which fertilisation is the 



