THE MECHANISTIC CONCEPTION OF LIFE 13 



special constituent of the nucleus, the chromosomes. The proof for 

 this was given by facts found along the lines of Mendelian investiga- 

 tions. The essential law of Mendel, the law of segregation, can in its 

 simplest form be expressed in the following way. If we cross two forms 

 which differ in only one character every hybrid resulting from this 

 union forms two kinds of sex-cells in equal numbers ; two kinds of eggs 

 if it is a female, two kinds of spermatozoa if it is a male. The one kind 

 corresponds to the pure paternal, the other to the pure maternal type. 

 The investigation of the structure and behavior of the nucleus showed 

 that the possibility for such a segregation of the sex-cells in a hybrid 

 can easily be recognized during a given stage in the formation of the 

 sex-cells, if the assumption is made that the chromosomes are the bear- 

 ers of the paternal characters. The proof for the correctness of this 

 view was furnished through the investigation of the heredity of those 

 qualities which occur mainly in one sex ; e. g., color blindness which oc- 

 curs preeminently in the male members of a family. 



Nine years ago McClung published a paper which solved the prob- 

 lem of sex determination, at least in its essential feature. Each animal 

 has a definite number of chromosomes in its cell nucleus. Henking 

 had found that in a certain form of insects (Pyrrhocoris) two kinds of 

 spermatozoa exist which differ in the fact that the one possesses a 

 nucleolus while the other does not. Montgomery afterwards showed that 

 Henking's nucleolus was an accessory chromosome. McClung first ex- 

 pressed the idea that this accessory chromosome was connected with the 

 determination of sex. Considering the importance of this idea we 

 may render it in his own words : 



A most significant fact, and one upon which almost all investigators are 

 united in opinion, is that the element is apportioned to but one half of the 

 spermatozoa. Assuming it to be true that the chromatin is the important part 

 of the cell in the matter of heredity, then it follows that we have two kinds of 

 spermatozoa that differ from each other in a vital matter. We expect, therefore, 

 to find in the offspring two sorts of individuals in approximately equal numbers, 

 under normal conditions, that exhibit marked differences in structure. A careful 

 consideration will suggest that nothing but sexual characters thus divides the 

 members of a species into two well-defined groups, and we are logically forced 

 to the conclusion that the peculiar chromosome has some bearing upon the 

 arrangement. 



I must here also point out a fact that does not seem to have the recognition 

 it deserves; viz., that if there is a cross division of the chromosomes in the 

 maturation mitoses, there must be two kinds of spermatozoa regardless of the 

 presence of the accessory chromosome. It is thus possible that even in the 

 absence of any specialized element a preponderant maleness would attach to one 

 half of the spermatozoa, due to the "qualitative division of the tetrads." 



The researches of the following years, especially the brilliant work 

 of E. B. Wilson, Miss Stevens, T. H. Morgan and others, have amply 

 confirmed the correctness of this ingenious idea and cleared up the 

 problem of sex determination in its main features. 



