nfectious 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.

[Illustration: FIG. 22.--DIAGRAMMATIC REPRESENTATION OF THE PROCESS OF
FERTILIZATION. (Boveri.) In the first cell (_a_) the ovum is shown in
process of fertilization by the entering spermatozoon or male sexual
element. In the following cells there is shown the increase in amount
of the male material and the final intimate commingling in _g_ which
precedes the first segmentation. _g_ represents a new organism formed
by the union of the male or female cell but differing from either of
them.]

Living matter is always individual, and this individuality is
expressed in slight structural variations from the type of the species
as shown in an average of measurements, and also in slight variations
in function or the reactions which living tissue shows towards the
conditions acting upon it. The anatomical variations are more striking
because they can be demonstrated by weight and measure, but the
functional variations are equally numerous. Thus, no two brains react
in exactly the same way to the impressions received by the sense
organs; there are differences in muscular action, differences in
digestion; these variations in function are due to variations in the
structure of living material which are too minute for our
comparatively coarse methods of detection. In the enormous complexity
of living matter it is impossible that there should not be minute
differences in molecular arrangement and to this such functional
variations may be due. Chemistry gives us a number of examples of
variations in the reaction of substances which with the same
composition differ in the molecular arrangement. Even in so simple a
mechanism as a watch there are slight differences in structure which
gives to each watch certain individual characteristics, but the type
as an instrument constructed for recording time remains. In the fusion
of the chromosomes of the male and female sexual cells, to which the
hereditary transmission of the ancestral qualities to the new
offspring is due, there are differences in the qualities of each, for
the individuality of the parents is expressed in the germ cells, and
the varying way in which these may fuse gives to the new cell
qualities of its own in