380 RECORDS. 



or extracted from the gray ; thirdly, since the proportion is 

 definite, that the extraction takes place by means of some defin- 

 ite mechanism. We are at present, I think, unable to imagine 

 an explanation of these truly astonishing facts save by the 

 assumption that the gray and white characters are borne in the 

 egg by corresponding discrete bodies or entities of some kind, 

 that may be mixed and unmixed without fusion, shuffled and 

 unshuffled like cards in a pack. The evidence is so far wholly 

 indirect, though I think none the less cogent. But now, bearing 

 in mind that the case of the gray and white mice is but a single 

 example of a widespread phenomenon, let us ask whether we 

 can actually find any definite structures in the egg, and particu- 

 larly in the nucleus, that may be assumed to represent such 

 entities. One of the most significant and remarkable discoveries 

 of modern biology is the fact that such entities exist, though it 

 is important not to forget that their significance in heredity is as 

 yet only an assumption, not a completely demonstrated fact. 



These entities are bodies known as " chromosomes," and are 

 represented in the diagrams by the rods in the nuclei.^ I can 

 not within the limits of this address attempt to do more than 

 touch on a few of the discoveries of recent years regarding the 

 chromosomes, though I think they may fairly be claimed to 

 constitute one of the most brilliant chapters in the whole history 

 of biology. The number of the chromosomes is constant in each 

 species and, only with a few exceptions of such a kind as to 

 emphasize the rule, the number in sexually produced organ- 

 isms is always an even one. It has been proved that during 

 the fertilization of the egg one half of the chromosomes are 

 derived from the father and one half from the mother (Fig. 3, ^), 

 and the still more suggestive fact has been established — with 

 probability through the study of normal development, with 

 almost complete 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 progeny are 



1 In point of fact the chromosomes are, as a rule, only distinctly visible at the 

 period of cell-division. In the diagram they are represented quite schematically, as 

 if visible in the resting nuclei. 



