no heredity of a disease because heredity in the mammal is solely a matter of the chromosomes and these could not convey a parasite.  The new organism can, however, quickly become diseased and, by the transference of disease to it and by either parent, there is the appearance of hereditary transmission of disease, though in reality it is not such.  The ovum itself can become the site of infection; this, which was first discovered by Pasteur in the eggs of silkworms, takes place not infrequently in the infection of insects with protozoa.  In Texas fever the ticks which transmit the disease, after filling with the infected blood, drop off and lay eggs which contain the parasites, and the disease is propagated by the young ticks in whom the parasites have multiplied.  The same thing is true in regard to the African relapsing or tick fever, which is also transferred by a tick.  In the white diarrhoea of chickens the eggs become infected before they are laid and the young chick is infected before it emerges from the shell.  It is highly improbable, and there is no certain evidence for it, that the extremely small amount of material contributed by the male can become infected and bring infection to the new organism.  In the cases in which disease of the male parent is transferred to the offspring, it is either by an infection of the female by the male, with transference of the infection from her to the developing organism, or with the male sexual cells there may be a transference to the female of the infectious material and the new organism may be directly infected.  No other disease in man is so easily and directly transferred from either parent to offspring as is syphilis, and the disease is extremely malignant for the foetus, usually causing death before the normal period of intra-uterine development is reached.

[Illustration:  FIG. 21.—­DIAGRAM SHOWING THE RELATION OF THE SEXUAL CELLS TO THE SOMATIC CELLS OR THOSE OF THE GENERAL BODY.  The sexual cells are represented to the left of the line at the bottom of diagram and are black.  From the fertilized ovum at the top there is a continuous cell development, with differentiation represented in the cell groups of the bottom row.  It is seen that the sexual cells are formed directly from the germ cell and contain no admixture from the cells of the body.]

The mother gives the protection of a narrow and unchanging environment and food to the new organism which develops within the uterus, and there is always a membranous separation between them.  Disease of the mother may affect the foetus in a number of ways.  In most cases the membrane of separation is an efficient guard preventing pathogenic organisms reaching the foetus from the mother.  In certain cases, however, the guard can be passed.  In smallpox, not infrequently, the disease extends from the mother to the foetus, and the child may die of the infection or be born at term with the scars resulting from the disease upon it.  Syphilis in the mother in an active stage is practically always extended to the foetus.  We have said that in an infectious disease substances of an injurious character are produced by bacteria, and such substances being in solution in the blood of the infected mother can pass through the membranous barrier and may destroy the foetus although the mother recovers from the infection.