CHAPTER X.
INHERITANCE AS A FACTOR IN DISEASE .-- THE PROCESS OF CELL MULTIPLICATION .-- THE SEXUAL CELLS DIFFER FROM THE OTHER CELLS OF THE BODY .-- INFECTION OF THE OVUM .-- INTRA-UTERINE INFECTION .-- THE PLACENTA AS A BARRIER TO INFECTION .-- VARIATIONS AND MUTATIONS .-- THE INHERITANCE OF SUSCEPTIBILITY TO DISEASE .-- THE INFLUENCE OF ALCOHOLISM IN THE PARENTS ON THE DESCENDANTS .-- THE HEREDITY OF NERVOUS DISEASES .-- TRANSMISSION OF DISEASE BY THE FEMALE ONLY .-- HEMOPHILIA .-- THE INHERITANCE OF MALFORMATIONS .-- THE CAUSES OF MALFORMATIONS .-- MATERNAL IMPRESSIONS HAVE NO INFLUENCE .-- EUGENICS.
The question of inheritance of disease is closely associated with the study of infection, and the general subject of heredity in its bearing on disease can be considered here.

By heredity is understood the transference of similar characteristics from one generation of organisms to another.

The formation of the sexual cells is a much more complex process than that of the formation of single differentiated cells, for the properties of all the cells of the body are represented in the sexual cells, to the union of which the heredity transmission of the qualities of the parents is due.

In the nucleus of all the cells in the body there is a material called _chromatin_, which in the process of cell division forms a convoluted thread; this afterwards divides into a number of loops called _chromosomes_, the number of which are constant for each animal species.

In cell division these loops divide longitudinally, one-half of each going to the two new cells which result from the division; each new cell has one-half of all the chromatin contained in the old and also one-half of the cytoplasm or the cell material outside of the nucleus.