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Page 48
[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.
[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 addition to qualities which come from each
ancestor, and from remote ancestors through these. The qualities with
which the new organism starts are those which it has received from its
ancestors plus its individuality. The fact that the sexual cells are
formed from the early formed cells of the new organism which represent
all of the qualities of the fertilized ovum or primordial cell,
renders it unlikely that the new offspring will contain qualities
which the parents have acquired. The question of the inheritance of
characteristics which the parents have acquired as the result of the
action of environment upon them is one which is still actively
investigated by the students of heredity, but the weight of evidence
is opposed to this belief.
In the new organism the type of the species is preserved and the
variations from the mean to which individuality is due are slight. We
are accustomed to regard as variations somewhat greater departures
from the species type than is represented in individuality, but there
is no sharp dividing line between them.
Very much wider departures from the species type are known as
mutations. Such variations and mutations, like individuality, may be
expressed in qualities which can be weighed and measured, or in
function, and all these can be inherited; certain of them known as
dominant characteristics more readily than others, which are known as
recessive. If these variations from the type are advantageous, they
may be preserved and become the property of the species, and it is in
this way that the characteristics of the different races have arisen.
Certain of the variations are unfavorable to the race. The varying
predisposition to infection which undoubtedly exists and may be
inherited represents such a variation. Tuberculosis is an instance of
this; for, while the cause of the disease is the tubercle bacillus,
there is enormous difference in the resistance of the body to its
action in different individuals. The disease is to a considerable
extent one of families, but while this is true the degree of the
influence exerted by heredity can be greatly overestimated. The
disease is so common that in tracing the ancestry of tuberculous
patients it is rare to find the disease not represented in the
ancestors. A further difficulty is that the environment is also
inherited. The child of a tuberculous parent has much better
opportunity to acquire the infection than a child without such an
environment [page 167]. Other diseases than the infectious seem to be
inherited, of which gout is an example. In gout there is an unusual
action of the cells of the body which leads to the formation and the
retention in the body of substances which are injurious. Here it is
not the disease which is inherited, but the variation in structure to
which the unusual and injurious action of the cells is due.
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