1. Epignesis Theory:
Wolff (1738-1794) who opposed the perforations concept opined that, neither the egg nor the sperm had a preformed organism, but both of them have undifferentiated mass of living substance which developed into the organism after fertilization.
According to this theory called epigenesis, new organs and tissues are formed denovo (from the mass of living substances in the gametes) due to vital forces.
2. Pangenesis Theory:
Hippocrates (400 BC) believed that reproductive material is produced from all parts of the body so that all the characters are represented in the progeny. Aristotle (350 BC) also was a believer of direct inheritance of characters.
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Charles Darwin (1809-1882) was also a votary for this type of inheritance. He modified the views of Hippocrates slightly and called it the theory of pangenesis.
According to this, all parts of the body produce invisible gemmules orpangenes which are carried into sex organs via the blood stream. There the pangenes are assembled into gametes. During fertilization, pangenes of mother and father combine to produce the characters of the offspring.
Lamarckian theory (J.B. Lamarck 1744-1829) of inheritance, often called inheritance of acquired characters opines that any new character acquired during the life time of an individual inherited. It will be seen from the above that Charles Darwin’s pangene theory is nothing but a version of Lamarckian theory.
3. Germplasm Theory of Weismann:
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August Weismann (1834- 1914) disproved the pangene theory with his experiments on rats. He cut the tails of rats and observed the progeny for 22 generations. In every generation the rat progeny continued to have tails in spite of the fact that the parents were tailless.
If pangene theory were to be correct, the tailless parents could not have sent the tail pangene to the gametes and consequently the progeny should have been tailless.
In order to account for inheritance, Weismann proposed his germplasm theory, which is essentially correct even today. According to this theory, the body of an organism can be divided into two – somatoplasm and germplasm.
Somatoplasm constitutes the vegetative body of the organism, while the germplasm constitutes the reproductive part. Somatoplasm develops newly in every individual and there is no continuity; while the germplasm is continuous and links the generations.
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With the information present in germplasm, every individual develops its own somatoplasm.
Hence any change or variation occurring in somatoplasm cannot be transmitted to the next generation, while those of germplams can be inherited.
The germplasm theory provided a convincing physical basis for distinction of variations into hereditary and environmental.