64 I'HK GERM-PLASM 



the germ-plasm are supplied to all the cells of the ontogeny, we 

 should have to suppose that differentiation of the body is due 

 to all the determinants except one particular one remaining 

 dormant in a regular order, and that, apart from special adapta- 

 tions, only one determinant reaches the cell, viz., that which has 

 to control it. This latter supposition is undoubtedly less likely 

 than the former. 



If however we do make this assumption, the question then 

 arises as to what factors can cause the gradual disintegration of 

 the id of germ-plasm into smaller and smaller groups of deter- 

 minants. — that is to say, into ids which contain fewer and fewer 

 kinds of determinants. 



This disintegration I believe to be due to the co-operation of 

 three factors : these are — t/ie inherited architecture of the gerni- 

 plasni. in which each determinant has its definite position ; the 

 iineqiially vigorous ninltiplication of the various determinants ; 

 and possibly also, the forces of attraction which are situated 

 within each determinant, and result from its specific nature as a 

 special and independent vital unit. The architecture of the 

 germ-plasm has already been discussed in general terms : for the 

 present, at any rate, we can hardly conjecture the actual details 

 of its structure. In order to do so, it would be necessary to sup- 

 pose that hundreds of thousands, or millions, of determinants, 

 which are all definitely localised, take part in the formation of 

 the higher organisms. The fact that the right and left halves 

 of the body can vary independently in bilaterally symmetrical 

 animals, points to the conclusion that all the determinants are 

 present in pairs in the germ-plasm. As, moreover, in many 

 of these animals, e.g. the frog, the division of the ovum into 

 the two first embryonic cells indicates a separation of the body 

 into right and left halves, it follows that the id of germ-plasm 

 itself possesses a bilateral structure, and that it also divides so 

 as to give rise to the determinants of the right and left halves of 

 the body. This illustration may be taken as a further proof of 

 our view of the constant architecture of the germ-plasm. An id 

 is evidently not constituted like the sediment of a complicated 

 and well-shaken mixture, in which the heavier particles come to 

 lie at the bottom and the lighter ones at the top ; nor is it con- 

 stituted in such a manner that the respective positions of the 

 particles are only determined independently by the forces acting 

 on them and between them momentarilv. Its stmcture mav be 



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