CHAPTER IV. 



THE REPRODUCTION AND LIFE HISTORY OF 

 ANIMALS. 



I. REPRODUCTION. 



IN the higher animals the beginnings of individual life are 

 hidden, within the womb in mammals, within the egg shell 

 in birds. It is natural, therefore, that early preoccupation 

 with those higher forms should have hindered the recog- 

 nition of what seems to us so evident, that almost every 

 animal arises from an egg cell or ovum which has been 

 fertilised by a male cell or spermatozoon. The exceptions 

 to this fact are those organisms which multiply by buds or 

 detached overgrowths, and those which arise from an egg 

 cell which requires no fertilisation. Thus Hydra may form 

 a separable bud, much as a rose bush sends out a sucker ; 

 thus drone bees " have a mother but no father," for they 

 arise from parthenogenetic eggs which are not fertilised. 

 Apart from these and similar cases, the "ovum theory," 

 which Agassiz called " the greatest discovery in the natural 

 sciences in modern times," is true, that each organism 

 begins from the division of a fertilised egg cell. 



History. We can realise this discovery better if we consider its 

 history. For a long time, on into the present century, what was called 

 the doctrine of prefonnation prevailed. According to this theory, 

 development was merely an unfolding ("evolution") of a preformed 

 miniature which lay within the germ. The " ovists " found this minia- 

 ture model of the future organism in the egg; the " animalculists " 

 found and even figured it within the spermatozoon. "There is no 

 becoming," said Haller, " no part of the body is made from another, all 

 are created at once." But this was not all. The germ was more than 

 a marvellous bud-like miniature of the adult, it included the next 

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