Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 edition. Excerpt: ... LECTURE XVII THE GERM-PLASM THEORY Conception of the 'id' deduced from the process of fertilization--Hereditary substance, 'idioplasm' and 'germ-plasm'--'Wants' -- Evolution or Epigenesis-- Herbert Spencer's uniform germinal substance--Determinants--Illustrations: Lyacna ajestis--The leaf-butterflies--Insect metamorphosis, limbs of segmented animals-- Heterotopia--The ultimate living units or biophors--Number of determinants-- Stridulating organ of the grasshopper. In proceeding to expound the theory of heredity which has shaped itself in my mind in the course of my own scientific development, I should like to begin by pointing out that the hereditary substance of the germ-cell of an animal or of a plant contains not only the primary constituents (Anlagen) of a single individual of the species, but rather those of several, often even of many individuals. That this is so can be proved in several ways. I start from what I hold to be the proved proposition, that the chromatin substance of the nucleus is the hereditary substance. We have seen that this is present in the germ-cells of every species in the form of a definite number of chromosomes, and that in germ-cells destined for fertilization, that is, in sex-cells, this number is first reduced to half, the reduction being effected, as is now proved in regard to a whole series of animals, by the two last cell-divisions, the so-called maturation divisions. We know that the full number is only reached again through amphimixis, by which process the half number of chromosomes in the male and female germ-cells are united in a single cell, the 'fertilized ovum, ' and in a single nucleus, the so-called segmentation nucleus. Thus the hereditary substance of the child is formed half from the..
