412 



Houj Animals and Flams Reproduce unit viii 



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Fig. 360 Paramecium dividing. Which is the later stage? What happens to the nu- 

 cleus? (general biological supply house) 



contents of the open flasks immediately 

 after the boiling, Pasteur found no trace 

 of bacteria. But after these bottles had 

 been in the laboratory for some time, he 

 found that they were full of organisms 

 which were decaying the food. In the 

 sealed bottles, however, there were no 

 signs of decay, Pasteur broke the seal of 

 a few flasks and let them stand open. 

 Very soon bacteria appeared in the flasks 

 and began to decay the food. Pasteur, 

 therefore, concluded that bacteria did 

 not develop from the lifeless food. He 

 decided that bacteria on the dust in the 

 air had settled in the open flasks. They 

 entered the liquid and reproduced until 

 there were very large numbers of them. 

 You can try this experiment for your- 

 self. Do Exercise i. 



To check his theory that the bacteria 

 entered the flasks with the dust from the 

 air, Pasteur took some of his flasks to the 

 top of a high mountain where the air 

 was almost free from dust. He found 

 that bacteria rarely appeared in the flasks 

 opened on the mountain top. 



Even after these experiments, there 

 were some who were not convinced that 

 bacteria always arose from other bacte- 

 ria. They thought it likely that lack of 

 air in the sealed flasks prevented the de- 



velopment of bacteria from the broth. 

 To check this final point Pasteur did an- 

 other experiment using several flasks of 

 a new kind. He prepared more broth ex- 

 actly like that used in previous experi- 

 ments. After heating the broth in a flask, 

 he heated the neck of the flask and drew 

 it out into a zigzag, open tube. Again 

 he heated the flasks, then set them aside. 

 Since the zigzag tube was open, air could 

 enter but no bacteria could drop in. No 

 bacteria were found in the flasks even 

 after four years. No objections to this 

 experiment were ever raised. 



Tyndall's experiments were different 

 in procedure but were equally convinc- 

 ing. Together the two men provided 

 proof that even the simplest known liv- 

 ing things arise only from other living 

 things. 



Reproduction of some simple organ- 

 isms. If you were to examine a rich cul- 

 ture of Paramecium (or similar proto- 

 zoan) you would find some of them 

 reproducing. Photographs of two stages 

 of the process may be seen in Figure 360. 

 Exercise 2 describes how to observe re- 

 production in Paramecium. 



It is not always easy to find an ameba 

 reproducing but the process has been de- 

 scribed fully. The ameba begins by pull- 



