MODES OF SEXUAL REPRODUCTION. 53 



favour the production of female offspring, with the opposite 

 conditions which favour the production of males; or by 

 contrasting the secondary sexual characters of the two sexes. 



Stages in the history of fertilisation. — While it is not difficult 

 to see the advantage of fertilisation as a process which helps to sustain 

 the standard or average of a species and as a source of new variations, 

 we can at present do little more than indicate various forms in which 

 the process occurs. 



(a) Formation of Plasmodia, the flowing together of numerous feeble 

 cells, as seen in the life-history of those very simple Protozoa 

 called Proteomyxa, e.g. Protomyxa, and Mycetozoa, e.g. flowers 

 of tan (Atlhalium septicum). 

 (6) Multiple conjugation, in which more than two cells unite and fuse 

 together, as in some Gregarines and in the sun-animalcule 

 (Actinosphcerium). 



(c) Ordinary conjugation, in which two similar cells fuse together, 



observed in Gregarines 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. 40, p. 93), 



(e) Fertilisation, in which a spermatozoon liberated from a Metazoon 



unites intimately with an ovum liberated from another individual 

 normally 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 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. Certain facts, such as 

 the occurrence of hermaphrodite organs as a transitory 

 stage in the development of the embryos of many 



