238 



THE AMERICAN MUSEUM JOURXAL 



Before passing on to the explanation 

 of what may be called the mechanism of 

 Mendelism, a word should l)e said for the 

 benefit of those who may have read or 

 hearfl the Mendelian principles gi\en in 

 terms of presence or absence of char- 

 acters. ^Ye may say that a fly's eye is 

 red in the presence of the factor for red, 

 and white in its absence, or we may speak 

 of the pair of characters as red and white. 

 It has seemed better to use the latter 

 alternative here, but the pre,sence-and- 

 absence way of putting it works out well 

 in certain cases and has given rise to some 

 interesting speculations. Thus, Profes- 

 sor Bateson has suggested that all or- 

 ganic evolution has been brought about 

 by the successive dropping out of char- 

 acters. This seems hard to believe, but 

 certainly the origin of many varieties, 

 whose origin we think we have seen, can 

 be neatly explained in that way. 



In order to understand the mechanism 

 of jVIendelian inheritance it will be neces- 

 sary to explain some of the details of cell 

 structure. The bodies of all the higher 

 animals and plants are made up of 

 cells, which are frequently looked upon 

 as units of body structure. The lowest 

 animals and plants consist of but one of 

 these cells. The germ cells, egg or sperm, 

 are merely some of these cells split off 

 from the main mass of body cells, and 

 differentiated so that they may unite 

 and form a new mass of body cells, the 

 new individual. In some cases the egg 

 cell can carry on this process without 

 uniting with the sperm, but in the vast 

 majority of cases among higher animals 

 and plants such imion is normally neces- 

 sary. Within these cells are bodies 

 called chromosomes, the name being 

 given because they stain deeply when 

 treated with certain reagents. The 

 chromosomes have, for some time, been 

 supposed to be the bearers of heritable 

 characters, and this supposition has now 



become almost a certainty by reason of 

 Mendelian studies, especially those with 

 the pomice fly, Dro.sophila arnpelophila. 

 We are, as yet, in the dark concerning 

 the exact method by which these char- 

 acters are transmitted, so that " bearers 

 of heritable characters" is in great part a 

 figure of speech, but, at any rate, these 

 characters are somehow bound up with 

 special chromosomes. 



]\Iost, and probably all, organisms 

 ha\e a definite number of these chromo- 

 somes, although the number is not 

 always the same in both sexes. In the 

 pomice fly the number is the same (eight) 

 in each sex, but one of the chromosomes 

 (the "Y") of the male seems to carry 

 maleness and not, as far as is known, any 

 other character. When it is present the 

 individual is a male. It is, however, 

 paired in the body cells of the males with 

 a chromosome which does carry factors 

 for certain body characters, and this 

 other chromosome may be called X. 

 In each of the female body cells there is a 

 pair of these X chromosomes but no Y. 

 When a body cell destined to become a 

 germ cell differentiates, the result of the 

 rather complicated process may be 

 stated simply by saying that it breaks 

 in two, making two nearly similar cells. 

 In the case of the male, the Y chromo- 

 some goes to one half, /. c to one sperm, 

 and the X chromosome to the other. 

 Each egg has> an X chromosome. If a 

 sperm having a Y chromosome enters an 

 egg, the union will have one X and one Y 

 and the resulting individual will be a 

 male. However, if a sperm having an 

 X chromosome enters an egg, the union 

 will have X paired with X; there will be 

 no Y and the resulting individual will be 

 a female. Since the chances are equal 

 that an egg will be fertilized by a Y- 

 bearing sperm or by an X-bearing sperm 

 the determination of sex is a random 

 matter; it depends upon which sperm 



