The process of sexual fertilization consists in the union of the male and female sex cells and an equal blending of the chromatin contained in each (Fig.

22).

In the process of formation of the sexual cells a diminution of the number of chromosomes contained in them takes place, but this is preceded by such an intimate intermingling of the chromatin that the sexual cells contain part of all the chromosomes of the undifferentiated cells from which they were formed.

The new cell which is formed by the union of the male and female sexual cells and which constitutes a new organism, contains the number of chromosomes characteristic of the species and parts of all the chromatin of the undifferentiated cells of male and female ancestors.

As a result of this the most complicated mechanism in nature, it is evident that in a strict sense there can be no heredity of a disease because heredity in the mammal is solely a matter of the chromosomes and these could not convey a parasite.