286 THE GERM-PLASM 



ber of animals the two first segmentation-cells of the egg, or 

 blastomeres. correspond to the future right and left halves of the 

 body. The first division of the nucleus of the fertilised ovum 

 must therefore separate the determinants for the right and left 

 halves of the body ; and the occurrence of this process is ren- 

 dered all the more probable from the fact that a longitudinal 

 division of the idants actually occurs, and results in each of the 

 spherical ids being halved. 



The explanation of the striking correspondence of the homolo- 

 gous parts of aiitimeroiis or paired organs must be referred to 

 the history of the transformation of species. The permanence 

 of this resemblance during the continued crossing of individuals, 

 is naturally due to this approximate similarity of the correspond- 

 ing determinants for the right and left sides in both parents. 

 It is evident, according to our conception of the structure of the 

 idioplasm, that the resemblance of the corresponding parts of 

 the child originates in the antimerous determinants of both par- 

 ents, for the latter determine the comparative number of homo- 

 dynamous ids on the paternal and maternal sides, together with 

 the relative ' controlling power ' of the idioplasm of the father 

 and mother in the organ in question. And since this must 

 be the same on both the right and left sides, the organ itself 

 must display the same combination of paternal and maternal 

 characters on either side : that is to say, its two sides must be 

 alike. 



I do not think that these facts can be understood by the aid 

 of any other theoretical assumption with regard to the stnicture 

 of the hereditary substance. The assumption of the existence of 

 pangenes, for example, might certainly explain the circumstance 

 that a combination of the paternal and maternal characteristics 

 does indeed occur in the organ in question, — in the external 

 ear. for instance, — but it cannot account for the fact that this 

 combination is similar in the right and left ears. 



These very facts seem to me to furnish a further welcome 

 proof of the correctness of the view arrived at by other methods, 

 that the hereditary combination in each part is predetermined 

 from the germ onwards. The right and left ears could not 

 possibly resemble each other, if the relative strength of the 

 hereditary tendencies on both sides were not predetermined for 

 all parts of the child by the nature of the paternal and maternal 

 idants. 



