EXERCISE VI 

 REPRODUCTION 



A. ABIOGENESIS 



Living organisms come into existence only from other organisms 

 through some form of reproduction. It was once supposed that living 

 things were sometimes produced directly out of non-living matter, an 

 old theory now referred to as dbiogenesis. 



The following experiment which is to be performed as a class demon- 

 stration represents in a simple way the kind of experiment by which 

 abiogenesis was disproved. Preparations for this experiment are to be 

 made as follows: Into each of several clean, sterilized petrie dishes or test- 

 tubes place a small amount of nutrient agar solution; put covers on the 

 petrie dishes and cotton plugs in the test-tubes and sterilize under 15-18 

 pounds steam pressure. One-half of the preparations are opened in the 

 presence of the class, exposing the agar to the air of the laboratory for 

 about fifteen minutes. The unexposed preparations are kept as controls. 



Examine the dishes at frequent intervals for signs of growing organ- 

 isms (molds, bacteria colonies, and yeasts). On which cultures 

 do they appear first? Source of the growths? Discuss possible sources 

 of contamination in the controls 1 if growths occur in them. 



B. TYPES OF REPRODUCTION 



Living organisms give rise to other organisms like themselves; that is, 

 they possess the power of reproduction. Since the life of the individual 

 is in every case limited, it is this reproductive capacity that prevents any 

 race from dying out. 



Reproduction may be of two general kinds: (1) asexual and (2) 

 sexual. It is the purpose of this exercise to determine as far as possible, 

 from a limited number of examples, the essential features of each of these 

 types of reproduction. 



A. Asexual Reproduction 

 1. Fission. 



la. Fission in a protozoon (Paramecium). Try to find living para- 

 mecia that are dividing by means of transverse constriction about the 

 middle, but do not spend much time in search. If living animals under- 



1 Note that air is not entirely excluded by the covers of the petrie dishes. A more 

 careful experiment is not needed, however, to illustrate the method of attacking 

 the theory^of abiogenesis. 



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