SUMMARY OF MODES OF REPRODUCTION. 51 



" body," and that they retain an organisation continuous in 

 quality with the original germ cell from which the parent 

 arose ; they are thus not very liable to be tainted by the 

 mishaps which may befall the " body " which bears them. 

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

 which have had different histories, the possibility of per- 

 mutations and combinations, in other words, of variation, is 

 evidently supplied. Thus it is not surprising to find that 

 the asexual method of liberating buds has been replaced in 

 most animals by the more economical and advantageous 

 process of sexual reproduction. 



Summary of Modes of Reproduction. 



A. In Single-celled Animals (Protozoa). 



(1) The almost mechanical rupture of an amceboid 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, 

 "free-cell formation," "endogenous multiplication" (e.g. in 

 Gregarines). 



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 liberated ; 



and other forms of asexual multiplication. 



(Sexual.) 



(a) The liberation of cells from a simple Metazoon, in which there is 

 so little division of labour that the distinction between body 

 cells and reproductive cells is not marked. (Hypothetical.) 



(i) The liberation of special reproductive or germ cells, which have 

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

 more or less unaltered, the inherent qualities of the original 

 germ cell from which. the parent arose. These special repro- 

 ductive cells — the ova and spermatozoa — are normally united in 

 fertilisation, but some animals have (parthenogenetic) ova which 

 develop without being fertilised. 



