44 



MANUAL OF ZOOLOGY. 



been long and closely disputed, and is far from being settled ; 

 so that it will be sufficient to indicate the facts upon which the 

 theory rests. 



If an animal or vegetable substance be soaked in hot or 

 cold water, so as to make an* organic infusion, and if this in- 

 fusion be exposed for a sufficient length of time to the air, the 

 following series of changes is usually observed : 



i. At the end of a longer or shorter time, there forms upon 

 the surface of the infusion a thin scum, or pellicle, which, 

 when examined microscopically, is found to consist of an in- 

 calculable number of extremely minute molecules (fig. 4, A). 





'*<- 



Fig. 4. A, Living particles or molecules developed in organic infusions ; B, Bacteria 

 developed in organic infusions, highly magnified. (After Beale.) 



2. In the next stage these molecules appear, many of them, 

 to have increased in size by endogenous division, till they 

 form short staff-shaped filaments, called "bacteria" (fig. 4, B). 

 Others increase in length by the same process until we get long 

 filamentous bodies produced, which are termed " v ibri nes '" 

 Both the bacteria and the vibrios now exhibit a vibratile or 

 serpentine movement through the surrounding fluid. 



3. After a varying period, the bacteria and vibrios become 

 motionless, and disintegrate so as to produce again a finely 

 molecular pellicle. 



4. Little spherical bodies may now appear, each of which is 

 provided with a vibratile cilium with which it moves actively 

 through the infusion (Monas lens). 



5. Varied forms of ciliated Infusoria some of which possess 

 a mouth and are otherwise highly organised may make their 

 appearance in the fluid. 



The above is the general sequence of the phenomena which 

 have been observed, and the following are the two theories 

 which have been advanced to account for them : 



a. By the advocates of spontaneous generation, "Abio- 

 genesis" or " Heterogeny," it is affirmed that the Infusoria 



