THE GERM-PLASM 85 



in the id of germ-plasm of those parts which vary correlatively. 

 It will be shown later on that local differences in nutrition occur 

 in the id, and that these may cause changes in the determinants 

 affected by them. If, now, the determinants controlling regions 

 of the body which are far apart, are situated close together in 

 the id, they might easily be affected simultaneously by influences 

 producing variation. But the perfectly definite architecture of 

 the id of germ-plasm, on which we base our argument, does not 

 only permit of a vicinity of the' determinants of parts of the 

 body far removed from one another, but actually requires it. 

 For, according to our assumption, the id of germ-plasm is not a 

 representation of the body in miniature, but a stiaicture of a 

 special kind, in which the individual component parts are 

 arranged in the order in which they are passed on subsequently 

 in the process of ontogeny to their final destination, viz., to the 

 determinates or hereditary parts. This however requires that the 

 determinants of the ectoderm should be closely adjacent to 

 those of the endoderm in the id, if they are to be distributed to 

 a primary ectoderm and a primary endoderm cell in the first 

 division of the ovum. A cell-division which leads to the 

 separation of widely differing groups of determinants, admits 

 of a close aggregation of these different groups in the id of 

 the mother-cell. This may give some slight insight into the 

 above-mentioned phenomena of correlation. 



7. The Magnitude of the Constituents of the 

 Germ-plasm 



The assumption that the germ-plasm is composed of biophors, 

 determinants, and ids, implies the existence within a narrow 

 space of a large number of ultimate vital units (biophors) 

 in all the higher organisms. The question arises whether a 

 sufficient number of these units can be contained within an id. 

 Although I believe it is at present quite impossible to obtain 

 anything like a reliable answer to this question by calculating 

 the relative sizes of the elements of the germ-plasm, it may 

 perhaps not be uninteresting to attempt to make such a calcu- 

 lation. 



In order to solve the problem with any approach to accuracy, 

 it would at least be necessary to know the sizes of a biophor and 



