RECORDS. 381 



distributed in equal number, step by step, to all the cells of the 

 body. The remarkable conclusion is thus reached that the 

 fertilized egg, and all the cells derived from it, contain a double 

 set of chromosomes, paternal and maternal (Fig. 3, D). The 

 no less interesting result has been experimentally reached that 

 either set — paternal or maternal — is sufficient for complete 

 development (^at least as far as the larval stages) ; for the tgg 

 may be caused to develop without the paternal chromosomes, 

 while conversely the paternal chromosomes alone will suffice 

 for the development of an Qgg from which the maternal nucleus 

 has been removed. Here for the first time we catch a glimpse 

 of the probable physical explanation of the phenomena of domi- 

 nance and recession that have of late so greatly aroused the 

 interest of experimenters on inheritance ; but above all, here is 

 found our first definite basis of observation for the assumption 

 that the nuclear organization is not merely a chemical or molec- 

 ular one, but represents beyond this some kind of definite ma- 

 terial configuration of the nuclear substance. 



The time will not allow me to do more than touch on the 

 very recent work that has confirmed and extended this conclu- 

 sion. It has been found, first, that in some species the chromo- 

 somes show constant differences of shape and size, which points 

 towards the conclusion that they may possess specific individual 

 characters. But beyond this indirect evidence, and quite inde- 

 pendently of it, Boveri has shown by direct experiments of great 

 ingenuity and beauty that qualitative physiological differences 

 among the chromosomes actually exist : for complete develop- 

 ment is only possible in the presence of a particular combination 

 of chromosomes. Hence the conclusion becomes probable that 

 there is a definite causal relation of some kind between the indi- 

 vidual chromosomes and the development of corresponding 

 characters or groups of characters ; or, in other words, that the 

 hereditary characters are in some manner distributed among the 

 chromosomes which form their physical basis in the Qgg. We 

 do not yet know in precisely what form this conclusion should be 

 formulated. We do not know, for instance, whether a single 

 unit-character, such as color, is determined by a single chromo- 



