FOOTNOTE:  [1] This was the case of a woman, by occupation a cook, whose numerous exchanges of service were accompanied by the appearance of cases of typhoid fever in the families.  This became so marked that an examination was made and she was found to be a typhoid carrier and as such constantly discharging typhoid bacilli.  She is now isolated.

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.  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