MULTIPLICATION AND DEVELOPMENT 59 



spontaneously generated in stagnant water or decaying 

 organic liquids was held by some naturalists until very 

 recent times. And it was not so easy to disprove the 

 assertions of such believers. If some water in which 

 there are apparently no living organisms, however 

 minute, be allowed to stand for a few days, it will 

 come to swarm with microscopic plants and animals. 

 Any organic liquid, as a broth or a vegetable infusion, 

 exposed to the air for a short time becomes foul through 

 the presence of innumerable microsccpic organisms. But 

 it has been certainly proved that these organisms are not 

 spontaneously produced in the water or organic fluid. 

 A few of them enter the water from the air, in which there 

 are always greater or less numbers of spores of micro- 

 scopic organisms. These spores germinate quickly when 

 they fall into water or some organic liquid, and the rapid 

 succession of generations soon gives rise to the hosts of 

 bacteria and one-celled animals which infest all standing 

 water. If all the active organisms and inactive spores in 

 a glass of water are killed by boiling the water, and this 

 sterilized water be put into a sterilized glass, and this 

 glass be so well closed that germs or spores cannot pass 

 from the air without into the sterilized liquid, no living 

 animals will ever appear in it. We know of no instance 

 of the spontaneous generation of animals, and all the 

 animals whose life-history we know are produced by other 

 animals of the same kind. 



Simplest multiplication and development. The sim- 

 plest method of multiplication and the simplest kind of 

 development shown among animals are exhibited by such 

 simple animals as Amccba and Paramaccium. The pro- 

 duction of new individuals is accomplished in Amoeba by 

 a simple division or fission of its body (a single cell) into 

 two practical ly equivalent parts. An Amccba which has 

 grown for some time contracts all of its finger-like 



