Febeuaky 24, 1905.] 



SCIENCE. 



291 



fairly be claimed to constitute one of the 

 most brilliant chapters in the whole history 

 of biology. The number of the chromo- 

 somes is constant in each species and, with 

 only a few exceptions of such a kind as to 

 emphasize the rule, the number in sexually 

 produced organisms is always an even one. 

 It has been proved that during the fertil- 

 ization of the egg one half of the chromo- 

 somes are derived from the father and one 

 half from the mother (Fig. 3, A), and the 

 still more suggestive fact has been estab- 

 lished—with probability through the study 

 of normal development, with almost com- 

 plete demonstration through the study of 

 hybrids — that at every division of the egg 

 the chromosomes also divide (Figs. 2, C, 

 3, B, C) in such a manner that their prog- 

 eny are 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 chromo- 

 somes, paternal and maternal (Fig. 3, P). 

 The no less interesting result has been ex- 

 perimentally reached that either set — pa- 

 ternal or maternal— is sufficient for com- 

 plete development (at least as far as the 

 larval stages) ; for the egg may be caused 

 to develop without the paternal chromo- 

 somes, while conversely the paternal chro- 

 mosomes alone will suffice for the develop- 

 ment of an egg from which the maternal 

 nucleus has been removed. Here for the 

 first time we catch a glim.pse of the prob- 

 able physical explanation of the phenomena 

 of dominance and recession that have of 

 late so greatly aroused the interest of ex- 

 perimenters on inheritance; but above all, 

 here is found our first definite basis of ob- 

 servation for the assumption that the nu- 

 clear organization is not merely a chemical 

 or molecular one, but represents beyond 

 this some kind of definite material eon- 

 figuration 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 chromosomes show constant dif- 

 ferences of shape and size, which points 

 towards the conclusion that they may pos- 

 sess specific individual characters. But 

 beyond this indirect evidence, and quite 

 independently of it, Boveri has shown by 

 direct experiments of great ingenuity and 

 beauty that qualitative physiological dif- 

 ferences among the chromosomes actually 

 exist ; for complete development 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 be- 

 tween the individual 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 chromo- 

 somes which form their physical basis in 

 the egg. We do not yet know in precisely 

 what form this conclusion should be for- 

 mulated. We do not know, for instance, 

 whether a single unit-character, such as 

 color, is determined by a single chromo- 

 some, or by a combination of chromosomes, 

 or whether this may vary in different cases. 

 In this direction we have taken but the first 

 uncertain steps towards a new horizon of 

 discovery. But the point I wish to em- 

 phasize is that if we admit such a distribu- 

 tion of characters among the chromosomes 

 in any measure and in any form, to just 

 this extent have we admitted the principle 

 of preformation as applied to the nuclear 

 substance or idioplasm. To this extent do 

 we admit, for example, that the physical 

 basis of inheritance in a frog's egg is not 

 simply a frog-determining substance, but 

 is, in close analogy with Bonnet's concep- 

 tion, a kind of original preformation or 

 microcosm, in which the individual frog- 



