e 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
i