IV.] FOUNDATION OF A TIIFORY OF HEREDITY. 1 83 



earlier date he had looked upon the coalescence of nuclei as 

 the most essential feature in the process of fertilization. He 

 now believes that this former opinion has been confirmed by 

 the recent discoveries which have been shortly described 

 above. 



Although I entirely agree with Hertwig, as far as the main 

 question is concerned, I cannot share his opinions when he 

 identifies Nageli's idioplasm with the nucleoplasm of the germ- 

 cell. Nageli's idioplasm certainly includes the germ-plasm, if 

 I may retain this expression for the sake of brevity. Nageli 

 in forming his hypothesis did indeed start with the germ-cells, 

 but his idioplasm not only represents the nucleoplasm of the 

 germ-cells, but also that of all the other cells of the organism ; 

 all these nucleoplasms taken together constitute Nageli's idio- 

 plasm. According to Nageli, the idioplasm forms a network 

 which extends through the whole body, and represents the 

 specific molecular basis which determines its nature. Although 

 this latter suggestion — the general part of his theory— is cer- 

 tainly valid, and although it is of great importance to have 

 originated the idea of idioplasm in this general sense, in 

 contrast to the somato-plasm (*' Nahrplasma '), it is nevertheless 

 true that we are not justified in retaining the details of his 

 theory. 



In the first place the idioplasm does not form a directly 

 continuous network throughout the entire body; and, secondl}', 

 the whole organism is not penetrated by a single substance of 

 homogeneous constitution, but each special kind of cell must 

 contain the specific idioplasm or nucleoplasm which determines 

 its nature. There are therefore in each organism a multitude 

 of different kinds of idioplasm. Thus we should be quite justi- 

 fied in generally speaking of Nageli's idioplasm as nucleoplasm, 

 and vice versa. 



It is perfectly certain that the idioplasm cannot form a con- 

 tinuous network through the whole organism, if it is seated in 

 the nucleus and not in the cell-body. Even if the bodies of 

 cells are everywhere connected by fine processes (as has been 

 proved in animals by Leydig and Heitzmann, and in plants by 

 various botanists), they do not form a network of idioplasm but 

 of somato-plasm ; a substance which, according to Nageli, 

 stands in marked contrast to idioplasm. Strasburger has 



