comparatively slight growth takes place. By about the twentieth week,
the house, it may be said, is set in order; and there follows a
period marked by the rapid growth of the fetus.

THE DEVELOPMENT OF FORM.--A very old explanation of embryonic
development was that the process consisted altogether in growth.
According to that view the embryo lay curled up in the egg; at the
outset it was equipped with organs, limbs, features, and all the
other bodily structures found in an adult. In order that the ovum
might be transformed into a mature infant, only unfolding and growth
were required. After the microscope came into use, however, so simple
an explanation could no longer be accepted. Scientists soon realized
that the embryo did not exist "ready made" in the ovum, which, even
when magnified, failed to bear the faintest likeness to a human
being.

Although the microscope made impossible this very simple explanation,
it gave in return a truer, if more complex, account of the
transformation from egg to offspring. By this means it has been
definitely proved that the ovum multiplies rapidly after it has been
fertilized, and becomes, as was explained in the preceding chapter, a
sac-like structure within which hangs a tiny clump of tissue. This
inner mass of cells forms the embryo.

It has proved a difficult task to secure very young human embryos,
and many of the ideas we hold relative to the initial stages in the
development of man are based upon what has been found true in certain
mammals, the class of animals to which we belong. The youngest human
ovum known at present has already undergone about two weeks'
development, and there the embryo is represented by a flat disk. From