52 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



is evidently supplied (see p. 63). Thus it is not surprising 

 to find that the asexual method of liberating buds has been 

 replaced in most animals by the liberation of special germ 

 cells, 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 amoeboid cell, which has 



become too large for physiological equilibrium (e.g., Schizogenes}. 



(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 Gre- 

 garines). 



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.) 



(b] 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 reproductive 

 cells the ova and spermatozoa are normally united in ferti- 

 lisation, but some animals have (parthenogenetic) ova which 

 develop without being fertilised. 



The Evolution of Sex. 



A further problem is to account for the two facts (a) that 

 most animals are either males or females, the former liberat- 

 ing actively motile male elements or spermatozoa, the latter 



