54 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



Stages in the History of Fertilisation. 



While it is not difficult to see the advantage of fertilisation as a pro- 

 cess 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 (ALthalium septicuui). 



(b) Multiple Conjugation, in which more than two cells unite and fuse 



together, as in some Gregarines and in the sun animalcule 

 (Actinosphtzrmm). 



(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. 23, p. 94). 



(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) Hermaphroditism 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 maleness 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," e.g., earthworm and leech, in 

 barnacles and acorn shells, in one species of oyster, in the 

 snail, and in many other Bivalves and Gastropods, in Tuni- 

 cates and in the hagfish. 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 spermatozoa 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 

 unisexual animals (e.g., frog and bird), make it likely that 

 hermaphroditism is the primitive condition, and that the 



