Germ Plasm

GERM PLASM, that part of the protoplasm which, according to the theory of Weismann (see Weismann's theory), is the carrier of hereditary properties. According to Weismann's view, the germ plasm is contained in the chromatin of the nucleus. The composition of the germ plasm, according to Weismann, is complex. It consists of the smallest particles-biofores; from their combination, determinants are obtained, which produce a specific type of cell in the development of the organism; determinants, combining, form ids, visible under the microscope as the smallest grains of chromosomes (chromomeres). Groups of ids constitute idants, represented by chromosomes. Ids include all determinants of the body of a given organism. In the chromosome, ids which can be obtained from different parents are arranged in a line. Thus, the germ plasm of Weismann, unlike the idioplasm of Nägeli (Naegeli), is contained, according to his theory, only in the sex cells. Therefore, only the latter can give rise to a new organism. This germ plasm is transmitted in its entirety during the division of the fertilized egg only to one of the two cleavage cells ("unequal inheritance" division), and this continues until the sex gland begins to form in the developing embryo. Its cells receive the entire germ plasm. The path which the germ plasm travels from the fertilized egg cell to the sex cells of the next generation was named by Weismann*-Keimbahn, the germ or germ cell line. In all other cells of the body, individual ids and determinants of the germ plasm are distributed, and different types of determinants determine the character of a given cell. Weismann's doctrine of the germ plasm succeeded the doctrine of Nägeli on idioplasm (see). In one respect, compared with his predecessor, Weismann approaches modern genetic views on the structural basis of heredity: he transfers hereditary structures from the vague concept of protoplasm to the nucleus and connects them with concretely existing chromosomes. Weismann's view of the chromosome of the fertilized egg as a linear series of separate units determining the hereditary properties of the organism already has, at least externally, a similarity with modern views of the Morganists, who represent the chromosome as a linear series of hereditary predispositions-genes. However, this similarity between Weismann's doctrine of the germ plasm and modern views on the structural basis of heredity ends, since Weismann's ids are all identical and each contains all the hereditary properties of a given species or individual, whereas according to the views of the Morganists, the genes that make up each chromosome are different, and each determines one or another hereditary trait of the organism, without also influencing other features of the developing phenotype. Combining modern views with Weismann's terminology, it can now be said that the chromosome is represented as a series of "determinants," with all chromosomes of a species being not identical "idants," but sharply different: each chromosome in the species complex is individual and consists of a series of determinants-genes peculiar only to it. In this, however, there is no complete similarity between the concept of "gene" and "determinant": according to modern views, the nuclei of all cells of the organism generally retain the same gene structure of the chromosomes as the fertilized egg; during the differentiation of somatic cells, genes are not themselves released from the chromosomes into the protoplasm, but probably send into the protoplasm certain enzyme-forming substances, perhaps of the same chemical composition as the genes themselves (R. Goldschmidt). It is these enzymes, developing in connection with the genes of the chromosomes, that can most likely be compared with the determinants and biofores of Weismann's theory. The theory of the germ cell line, developed by Weismann, found brilliant confirmation in the work of Boveri, who showed that in the horse ascaris during cleavage, the fine structure of the fertilized egg is transmitted completely only to the germ cells, while the body cells undergo a special process of "diminution" of chromatin, after which the large chromosomes of the germ cells break down into a series of grains. The result is the conception that somatic cells, after such diminution, lose the properties of the complete germ plasm and cannot give rise to the development of a complete organism. However, in other cases, there is no such difference between germ cells and at least some body cells, which explains the phenomena of regeneration and asexual reproduction. To explain the latter, Weismann had to build complex additional superstructures to his theory of the germ plasm. The doctrine of the non-inheritability of changes that occur in the body of an organism during individual development, which is a conclusion from the theory of the germ plasm, forms the basis of modern genetics.