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DEFINITION

This is the theory that postulates that living organisms develop from non-living things

THOSE WHO BELIEVE IN SPONATNEOUS GENERATION

The great Greek philosopher Aristotle (384–322 BCE) believed in spontaneous generation.  He thought that some simpler invertebrates such as housefly and worm could arise by spontaneous generation.

Leeuwenhoek’s discovery of microorganisms by using the Microscope renewed the controversy. Some proposed that microorganisms arose by spontaneous generation even though larger organisms did not.

They pointed out that boiled extracts of hay or meat gave rise to microorganisms after sitting for a while.

In 1748 the English priest John Needham (1713–1781) reported the results of his experiments on spontaneous generation. Needham boiled mutton broth in flasks that he then tightly stoppered. 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 nonliving matter.

Lazzaro Spallanzani (1729–1799) 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 three-quarters 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 growth of animals already in the medium. The supporters of spontaneous generation maintained that heating the air in sealed flasks destroyed its ability to support life. Several investigators attempted to counter such arguments

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.

Subsequently

Georg Friedrich Schroder (1810–1885) and Theodor von Dusch (1824–1890) allowed 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. Despite these experiments, the French naturalist Felix Pouchet (1800–1872) claimed in 1859 to have carried out experiments conclusively proving that microbial growth could occur without air contamination

THOSE WHO DISPROVED SPONTANEOUS MUTATION

Francesco Redi (1626–1697), the Italian physician 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, a second was covered with paper, and the third was

covered with fine gauze that would exclude flies. Flies laid their eggs 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 eggs 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.

Louis Pasteur (1822–1895) settled the matter.

He 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. After which, he placed nutrient solutions in flasks, heated their necks in a flame, and drew them out into a variety of curves. The swan neck flasks that he produced

in this way had necks open to the atmosphere. Pasteur then boiled the solutions for a 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. However, Pasteur had not only resolved the controversy by 1861 but also had shown how to keep solutions sterile.

The English physicist John Tyndall (1820–1893) and the German botanist Ferdinand Cohn (1828–1898) dealt a final blow to spontaneous generation. In 1877 Tyndall demonstrated

that dust did indeed carry germs and that if dust was absent, broth remained sterile even if directly exposed to air. During the course of his studies, Tyndall provided evidence for the

existence of exceptionally heat-resistant forms of bacteria. Working independently, Cohn discovered that the heat- resistant bacteria recognized by Tyndall were species capable of producing bacterial endospores. Cohn later played an instrumental role in establishing a classification system for procaryotes based on their morphology and physiology.

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