Weismann’s Germ plasm theory
August weismann (1834-1914) was a neo-darwiniam biologists who Proposed the Germ plasm theory in his book “Daskeim plasma”. He said that variations are of two types. Some are congenital that is organisms are born with them. Others are acquired during the life time of a particular plant or animal. It was with this latter type of variations,the acquired characters,that weismann was very much concerned. He gradually developed his theory of Germplasm to explain that acquired characters could not be inherited.
The main points of Germ plasm theory are the following…..
(1) Existence of somatoplasm and germplasm:-
All substance of an organism can be divided into two parts, the germplasm (which is the protoplasm of germ cells such as sperms and ova)and somatoplasm (which is the protoplasm of somatic or body cells). The germplasm is thought to be the actual vehicle of heredity, it is not affected by any influence either from the body or the external environment. The somatoplasm is thought not to play any role in heridity.
(2) continuity of germplasm:-
In reproduction, a portion of Germplasm is derived from each parent, the paternal serm and maternal ovum, each of which has an equivalent share in the inheritance. These combine to form the fertilised egg. The contents of egg divided and re divided to produce both body (soma) and germplasm of the new individuals. Germplasm is immortal since it is perpetuated from generation to generation through meiotic cell division. Germplasm can produce somatoplasm but somatoplasm can not produce the germplasm.
(3) Architecture of germplasm:-
In 1904, weismann proposed that every distinct part of an organism is represented in the sex cells by a separate particle, the idiom plasm or determinant. The complete set of determiners would be handed down generation to generation,which would account for hereditary transmission of characters. The determines according to weismann are localised in the chromosome of the nucleus,just as are the genes of modern genetics.
(4) Struggle among determinants:-
weismann believed that within each organism occurs an internal stimulus which affects germ cells and result in heritable variation. According to weismann, nourishment is necessary for determinant or are fast growing and stronger than those getting less nourishment. He assumed a struggle for existence between better nourished and less nourished determinants and there lies the causes of appearance and disappearance of variation.