EFFECrS OF AMPHIMIXIS ON ONTOGENY 265 



differetit from one another, and does not consist of a number of 

 identical ids ; and that the cell may take on an intermediate 

 character in consequence of their co-operation. In normal repro- 

 duction the active ids of the idioplasm are all homologous. — that 

 is to say, their function is to control the same part of the body, 

 — but thev differ among themselves, i.e., are heterodynamous, 

 or, in other words, they tend to impress a somewhat ditTerent 

 character on the same part of the body. In the crosses between 

 different species, the idioplasm of a cell in many stages will be 

 composed of homologous as well as of heterologous ids, and, 

 as has just been shown, it then remains to be decided whether 

 a common control of the cell is possible at all, and if so, over 

 how many generations of cells it can extend. 



The scales of butterflies are cells belonging to the final stage of 

 ontogeny, and their ids are made up of determinants of one kind 

 only. The further back we pass towards the beginning of on- 

 togeny, the more numerous are the determinants composing the 

 ids ; onlv one of them, however, breaks up into biophors and 

 controls the cell. If the determinant which actually controls the 

 cell is homologous in both parents, an intermediate form of cell 

 may result ; but as soon as the rest of the determinants in the id 

 no longer correspond to one another, the further development 

 gradually becomes checked, and will ultimately be brought to a 

 stand-still. The processes which must be supposed to take 

 place from the beginning of ontogeny onwards in the case of the 

 supposed cross between a sea-urchin and a worm may very 

 possibly be only partial, i.e., they may refer merely to special 

 parts. Let us suppose that a certain species of insect, which is 

 normal in other respects, possesses limbs on the dorsal side 

 instead of wings, and that this is crossed with another insect 

 possessing normal wings, development of the fertilised egg 

 taking place up to the stage in which the wings arise. The 

 idioplasm of the primary cell of the wings and ' dorsal limbs ' 

 respectivelv would then contain two perfectly heterologous ids, 

 one derived from the father and the other from the mother, and 

 none of the determinants in these two ids would be respectively 

 homologous. In the first stage of the development of the wings 

 or ' dorsal limbs.' as the case may be. antagonistic determinants 

 would be opposed to one another in each cell-generation, and 

 would prevent a common determination of the cell. 



Such extreme cases do not actually occur, as verv different 



