CHAP, xii The Life- History of Animals 189 



kinds of cells are usually formed at different times, and 

 that the fertilisation of ova by spermatozoa from the same 

 animal very rarely occurs. It is very likely that the 

 bisexual or hermaphrodite state of periodic maleness and 

 femaleness is more primitive than that of separate sexes, 

 which, except in tunicates, a few fishes and amphibians, 

 and casual abnormalities, is constant among the backboned 

 animals. 



(b) Parthenogenesis seems to be a degenerate form of 

 sexual reproduction in which the ova produced by female 

 organisms develop without being fertilised by male cells. 

 Thus " the drones have a mother but no father," for they 

 develop from ova which are not fertilised. In some rotifers 

 the males have never been found, and yet the fertility of the 

 females is very great ; in many small crustaceans (" water- 

 fleas ") the males seem to die off and are unrepresented for 

 long periods ; in the aphides males may be absent for a 

 summer (or in a greenhouse for years) without affecting the 

 rapid succession of female generations. 



(c) Alternation of Generations. A fixed asexual zoophyte 

 or hydroid sometimes buds off and liberates sexual swim- 

 ming bells or medusoids, whose fertilised ova develop into 

 embryos which settle down and grow into hydroids. This is 

 perhaps the simplest and clearest illustration of alternation 

 of generations. 



In autumn the freshwater sponge (Spongilla) begins 

 to suffer from the cold and the scarcity of food. It dies 

 away ; but some of the units club together to form 

 " gemmules " from which in spring male and female 

 sponges are developed. The males are short-lived, but 

 their spermatozoa fertilise the ova of the females. The 

 fertilised ovum develops into a ciliated embryo, and this 

 into the asexual sponge, which produces the gemmules. 



The large free-swimming and sexual jellyfishes of the 

 genus Aurelia produce ova and spermatozoa ; from the 

 fertilised ovum an embryo develops not into a jellyfish, but 

 into a sessile Hydra-\f&.e animal. This grows and divides 

 and gives origin asexually to jellyfish. 



Similar but sometimes more complicated alternations 



