62 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



And, finally, in the mixture of two units of living matter 

 which have had different histories, an opportunity for new 

 permutations and combinations, in other words, for 

 variation, is supplied. Thus it is not surprising to find 

 that the asexual method of liberating buds has been re- 

 placed in most animals by the more economical and ad- 

 vantageous process of sexual reproduction. 



Summary of Modes of Reproduction 



A. In Single-celled Animals (Protozoa) 



(i) The almost mechanical rupture of an amoeboid cell, which has 

 become too large for physiological equilibrium. 



(2) The discharge of numerous superficial buds at once {e.g. Arcella 



and Pelomyxa). 



(3) The formation of one bud at a time (very common). 



(4) The ordinary division into two daughter cells at the limit of 



growth. 



(5) Repeated divisions within limited time and within limited space 



(a cyst). This results in what is called spore-formation {e.g. 

 in Sporozoa). 



B. In Many-celled Animals (Metazoa) 



{Asexual) 



{a) The separation of a clump of body cells, e.g. from the surface of 



some Sponges. (A crude form of budding.) 

 {b) The formation of definite buds which may or may not be set free, 

 (c) Various forms of fission and fragmentation. 



{Sexual) 



The liberation of special reproductive or germ cells, which have 

 not taken part in the formation of the body, and which retain 

 the essential qualities of the original germ cell from which the 

 parent arose. These special germ cells — the ova and sperma- 

 tozoa — are normally united in fertilisation, but some animals 

 have (parthenogenetic) ova which develop without being 

 fertilised. 



Evolution of sex. — A further problem is to account for 

 the two facts — {a) that most animals are either males or 

 females, the former liberating actively motile male elements 

 or spermatozoa, the latter forming and usually liberating 



