HERITAGE OF THE INDIVIDUAL 287 



an ever-increasing number of facts in genetics. Only the 

 future can determine whether they are universal. 



E. MECHANISM OF MEND ELIAN INHERITANCE 



With this general outline of the Mendelian principles 

 before us, it is now necessary to bring them into relation with 

 the facts so far known in regard to the structure of the germ 

 cells. In other words, we have assumed germinal factors, 

 or genes, segregation, etc., but has the actual study of cells 

 (cytology) given any evidence of the physical basis of genes 

 and of a segregating mechanism? The reader will at once 

 answer this in the affirmative on the basis of our discussion 

 of the origin and structure of the germ cells and their behavior 

 in fertilization. But all, or nearly all, of these cardinal facts 

 were unknown when Mendel worked and this makes still 

 more remarkable his prevision in interpreting his results in 

 the terms he did. 



The essential facts may now be restated from a slightly 

 different viewpoint. The egg and sperm each carry a definite 

 number of chromosomes and consequently after fertilization 

 the zygote contains a double set. For each chromosome con- 

 tributed by the sperm there is a corresponding, or homolo- 

 gous, chromosome contributed by the egg. In other words, 

 there are two chromosomes of each kind which may be con- 

 sidered as pairs. When division of the zygote takes place each 

 chromosome splits into two chromosomes, so that each 

 daughter cell receives a daughter chromosome derived from 

 each of the original ones. Since all the cells of the organism 

 are lineal descendants by similar mitotic cell divisions, all of 

 its cells contain the double set of chromosomes half paternal 

 and half maternal; and since the primordial germ cells have 

 a similar origin, they also have a double set of chromosomes. 

 But during the maturation process synapsis occurs: that is, 



