XI.] PROBLEMS OF THE DAY. 83 



I contrasted the body {soma) with the germ-cells and explained 

 heredity by the supposition of a material basis residing in the 

 germ-cells ; i. e. the germ-plasm, which is continuously passed 

 on from one generation to another. When the essay was being 

 written, I was not aware that this germ-plasm existed only in 

 the nucleus of the egg-cell, and I was therefore able to contrast the 

 entire substance of which the egg-cell consists, orthegerm-plasm, 

 with the substance which composes the body-cells, hence called 

 somatoplasm. In the fourth essay ^ (1885) I expressed my con- 

 viction, which agreed with that shortly before expressed by 

 Strasburger and O. Hertwig, that the substance of the egg- 

 nucleus, or, more precisely, the chromatin of the nuclear loops, 

 formed the material basis of heredity, the body of the cell being 

 only nutritive and capable of being moulded by forces ema- 

 nating from the nucleus, but in no way formative. Together 

 with the two above-mentioned writers, I transferred the con- 

 ception of idioplasm — introduced at that time by Nageli, although 

 defined by him in an essentially different manner, — to the ma- 

 terial basis of heredity in the egg-nucleus, and submitted that 

 not only in the ovum but in every cell the chromatin of the 

 nuclear thread was the idioplasm which dominated the whole 

 cell, and impressed its own specific character upon the origin- 

 ally indifferent cell-body. From this time I no longer spoke of 

 the cells of the body as simply somatic protoplasm (somato- 

 plasm), but in each cell I distinguished, first, between the 

 idioplasm, or substance which gives to the nucleus its power 

 of predisposition, and the body of the cell or cytoplasm; and, 

 secondly, I distinguished between the idioplasm of the egg- 

 nucleus and that of the nucleus of somatic cells. The idioplasm 

 of the germ- or sperm-cell alone was called germ-plasm (idio- 

 plasm of reproductive cells), while the idioplasm of the somatic 

 cells was called somatic idioplasm. Embryogeny, in my opinion, 

 depends only on changes in the idioplasm of the egg-nucleus, 

 i. e. changes in the germ-plasm. In my fourth essay there is a 

 description of the manner in which the idioplasm of the egg- 

 nucleus divides, in many species, at the first segmenta- 

 tion, each half undergoing certain regular modifications of 

 nuclear substance, so that neither daughter-cell possesses 

 the collective hereditary tendencies of the species, but one 



^ See Vol. I, p. 163. 



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