Bearers of Hereditary Characters 199 



thus carefully divided and evidently perpetuated without 

 alteration. Furthermore, nothing else can be discovered 

 in the cell which is so carefully divided and perpetuated 

 without alteration. It is natural to conclude, therefore, 

 that chromosomes are the bearers of hereditary char- 

 acters, for if these bearers were anything that is not so 

 carefully divided inheritance would be chaos instead 

 of the remarkable cosmos it is. 



(2) The behavior of chromosomes in the reduction 

 division is a remarkable piece of evidence. The fun- 

 damental features of this division are so familiar that 

 they need not be repeated. It is sufficient to say that 

 reduction division supplies a remarkable mechanism 

 which explains perfectly Mendel's theory of segrega- 

 tion. This remarkable coincidence of the observed 

 cytological facts with the preconceived theoretical 

 explanation of inheritance is probably the strongest 

 argument in favor of the chromosome hypothesis. 



(3) An argument may also be obtained from the 

 cytology of fertilization. In the higher plants the 

 contributions of male and female to the structure of 

 the zygote are as follows. The egg contributes a 

 nucleus plus a portion of the cytoplasm in the embryo 

 sac, which may well contain many types of cell con- 

 stituents. The male, however, contributes only the 

 nucleus stripped of its cytoplasm. It is safe to infer 

 that the male contributes quite as much to the off- 

 spring as the female, at least in the matter of hereditary 

 characters, so that the conclusion may be that hereditary 

 characters are carried entirely in the nucleus. Many 

 cytologists claim that the nucleus is entirely made up of 

 chromatin, and since chromatin is the material of 



