LECTURE XVIII 

 THE GERM-PLASM THEORY (continued) 



structure of the germ-plasm — Vital affinities — Division — 0. Hertwig's chief objec- 

 tions to this theory — Male and female eggs in the Phylloxera show differential division 

 — Dispersal of the germ-plasm in the course of Ontogeny — Active and passive state 

 of the determinants — Predetermination of cells — ^ There are no determinants of 

 characters — Liberation of the determinants — Accessory idioplasm — Hei-bst's lithium 

 larvae— Plant galls— Cells with several facultative determinants— Connective tissue in 

 vertebrates — Mesoderm cells of Echinoderms — Sexual dimorphism — Female and male 

 ids — Polymorphism (Papilio merope) — Ants. 



I HAVE endeavoured to prove that the germ-substance proper 

 must be looked for in the chromatin of the nucleus of the germ-cell, 

 and more precisely still in those ids or chromosomes which we con- 

 ceive of as containing the primary constituents ( Anlagen ) of a 

 complete organism. Such ids in larger or smaller numbers make up 

 the whole germ-plasm of a germ-cell, and each id in its turn consists 

 of primary constituents or determinants, i.e. of vital units, each of which 

 determines the origin and development of a particular part of the 

 organism. We have now to make an attempt to picture to ourselves 

 how these determinants predetermine those cells or cell-groups to 

 which they correspond. In doing so we have to fall back upon mere 

 hypotheses, and in stating any such hypothesis I wish expressly to 

 emphasize that I am only following up one of the possibilities which 

 our imaginative faculty suggests. Nevertheless, to endeavour to 

 form such a conception is certainly not without use, for it is only by 

 elaborating a theory to the utmost that we are able to use it in 

 application to concrete cases, thus stimulating the search for corro- 

 boratory or contradictory facts, and leading gradually to a recognition 

 of the gaps or mistakes in the theory. 



The first condition that must be fulfilled in order that a deter- 

 minant may be able to control a cell or cell-group is that it should 

 succeed in getting into it. It must be guided through the numerous 

 cell-divisions of ontogeny so that it shall ultimately come to lie in the 

 cells which it is to control. This presupposes that each determinant 

 has from the very beginning its definite position in relation to the 

 rest, and that the germ-plasm, therefore, is not a mere loose aggregate 

 of determinants, but that it possesses a structure, an architecture, in 

 I. A a 



