isease 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 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 th