SPONTANEOUS GENERATION

 

From earliest times, people had believed in Spontaneous generation that living organisms could develop from nonliving matter. Even Aristotle (384-322 B.C.) thought some of the simpler invertebrates could arise by non living matter.

• Francesco Redi (1626-1697) the Italian physician discredit the theory by carried out a series of experiments on decaying meat and its ability to produce maggots spontaneously. 
• Redi placed meat in three containers. One was uncovered, second was covered with paper and the third was covered with a fine gauze that would exclude flies. Flies laid their egg on the uncovered meat and maggots developed. 
• The other two pieces of meat did not produce maggots spontaneously. However, flies were attracted to the gauze covered container and laid their egg on the gauze; these eggs produced maggots.
• Thus the generation of maggots by decaying meat resulted from the presence of fly eggs, and meat did not spontaneously generate maggots as previously believed. Similar experiments by others helped discredit the theory for larger organisms.

• Leeuwenhoek's discovery of microorganisms renewed the controversy. Some investigators proposed that microorganisms arose by spontaneous generation even though larger organisms did not. 
• They pointed out that boiled extracts of hay or meat would give rise to microorganisms after sitting for a while.
• In 1748 John Needham (1713-1781) reported the results of his experiments on spontaneous generation. Needham boiled mutton broth and then tightly stoppered the flasks. Eventually many of the flasks became cloudy and contained microorganisms. He thought organic matter contained a vital force that could confer the properties of life on living matter.

• Lazzaro Spallanzani (1729-1799) the Italian priest and naturalist improved on Needham's experimental design by first sealing glass flasks that contained water and seeds. If the sealed flasks were placed in boiling water for ¾ of an hour, no growth took place as long as the flasks remained sealed. He proposed that air carried germs to the culture medium, but also commented that the external air might be required for the growth of animals already in the medium.

• Theodore Schwann (1810-1882) allowed air to enter a flask containing a sterile nutrient solution after the air had passed through a red hot tube. The flask remained sterile.

• Georg Friedrich Schroder and Theodor von Duschallowed air to enter a flask of heat sterilized medium after it had passed through sterile cotton wool. No growth occurred in the medium even though the air had not been heated.

• In 1859 Felix Pouchet concluded by his experiments that microbial growth occur without air contamination. This conclusion provoked Louis Pasteur (1822-1895) to settle the matter. He first filtered air through cotton and found that objects resembling plant spores had been trapped. If a piece of the cotton was placed in sterile medium after air had been filtered through it, microbial growth occurred.

• Next he placed nutrient solution in flasks heated their necks in a flame, keeping the ends of the necks open to the atmosphere. Pasture then boiled the solutions for few minutes and allowed them to cool. No growth took place even though the contents of the flasks were exposed to the air. Pasteur pointed out that no growth occurred because dust and germs had been trapped on the walls of the curved necks. If the necks were broken, growth commenced immediately.

• John Tyndall (1820-1893)dealt a final blow to spontaneous generation in 1877 by demonstrating that dust did indeed carry germs, if dust was absent broth remained sterile even directly exposed to air.