LECTURE XVII 

 THE GERM-PLASM THEORY 



Conception of the 'id' deduced from the process of fertilization— Hereditarj 



substance, 'idioplasm' and 'germ-plasm' — ' Idants ' — Evolution or Epif 



Herbert Spencer's uniform germinal substance — Determinants — Illustrations 

 agestis — 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 develop- 

 ment, 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 paternal, half from the maternal hereditary substance, and 

 we have seen that this remains so during the whole development 

 of the child, since, at every succeeding cell-division each of the 

 paternal and each of the maternal chromosomes doubles by dividing 

 and the resulting halves are distributed between the two daughter- 

 nuclei. 



Now if the complete hereditary substance of a germ-cell before 

 the reducing divisions contains potentially all the primary constituents 



