16 The Mechanistic Conception of Life 



the chromosomes. The proof for this was given by facts found 

 along the lines of Mendelian investigations. 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 chromo- 

 somes are the bearers 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 occurs pre- 

 eminently in the male members of a family. 



Nine years ago McClung published a paper which solved 

 the problem 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 afterward showed 

 that Henking's nucleolus was an accessory chromosome. 

 McClung first expressed the idea that this accessory chro- 

 mosome was connected with the determination of sex. Con- 

 sidering the importance of this idea we may render it in his 

 own words: 



A most significant fact, and one upon which almost all investi- 

 gators 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, 



