NATURE OF REPRODUCTION. 681 



bacterium cells, often with extreme velocity, through the fluid in which 

 they are immersed. 



Of later years, the investigations in regard to spontaneous generation 

 have been almost exclusively confined to the bacteria and their allies, 

 since they now form the only group of organisms in which reproduction 

 by generation has not been fully established. Even for them, the rapid 

 multiplication by cell division, which takes place under favorable con- 

 ditions, indicates the usual mode of their increase in numbers; but in 

 order to establish an entire similarity between them and other living 

 organisms, they must also be shown to reproduce themselves by spores 

 or germs, which has not thus far been done. The experiments with 

 boiled infusions in sealed flasks have led to results which are not inter- 

 preted in the same manner by all writers ; but it is evident that for 

 bacteria, as well as for other organic forms, the application of heat 

 exerts in various degrees a preventive action on their subsequent appear 

 ance. 



Among the most careful and satisfactory experiments on this part of 

 the subject are those of Prof. Wyman, 1 who operated with infusions of 

 both animal and vegetable matters. The infusions, placed in sealed 

 flasks, with abundance of atmospheric air, were submerged in boiling 

 water for periods varying from thirty minutes to five hours, and after- 

 ward kept under observation at the ordinary temperatures requisite for 

 the development of bacteria. The result showed that the appearance 

 of these organisms was always delayed by the previous application of 

 heat, and that this dela}^ in different series of observations, was often in 

 direct proportion to the length of time during which the boiling had 

 been continued. Furthermore, in certain cases the bacteria failed to be 

 produced at all, and tlu; chances of their production decreased in pro- 

 portion to the length of time during which the liquid had been boiled. 

 Thus, of four series of flasks, each containing the same infusion, and 

 boiled respectively during one, two, three, and four hours, all of the first 

 and second series afterward produced bacteria, only one of the third, 

 and none of the fourth. Finally, in no instance, among numerous trials, 

 did they appear in any infusion which had been boiled for a period ex- 

 ceeding five hours. Thus a limit was reached to the production of bac- 

 teria, in fluids previously subjected to the action of heat. 



There can be no doubt as to the scientific bearing of these and similar 

 experiments. Spontaneous generation is inadmissible at the present 

 day for everything except bacteria ; and with regard to them there is 

 no sufficient proof that they are ever generated without the concurrence 

 of previously existing germs. 



1 American Journal of Science and Arts. New Haven, vol. xliv., September, 

 1867. 



44 



