epresents a chapter in the life history of every individual so
important in its relations to maternity and paternity that every young
man should be acquainted with at least its general features.

As stated in the chapter on Reproduction, every living organism begins
life as a single cell, or globule of protoplasm. In the case of the
human subject, the cell from which each child begins its development
is formed by the fusion of two cells or globules of protoplasm, one
furnished by the mother, and called the ovum, or egg; the other
furnished by the father, and called the spermatozoon. The egg is very
much larger than the spermatozoon, and contains enough yolk material
to afford nourishment for the embryo for a number of days.

When the ovum reaches the finished state, which is called "maturity,"
it leaves the ovary, and is carried along the fallopian tube (see
accompanying figure) into the uterus, where it usually finds a
lodgment in the upper part, as shown in Figure I. Once the minute ovum
has been caught in the projections of the velvety inner surface of the
uterus, this thick velvety lining of the uterus in the neighborhood of
the ovum begins a rapid growth, gradually enveloping the rapidly
expanding ovum, as shown in Figures I and II of the accompanying
plate.

Within the ovum there are taking place some of the most marvelous
changes in the whole life history of the individual. The nucleus of
the fertilized egg, and the protoplasm which surrounds it, divide into
two cells, then into four, eight, sixteen, etc. These divisions follow
each other in such rapid succession that there are many hundreds of
cells by the end of the first twenty-four hours. These cells soon
begin to arrange themselves into layers and groups, which, step by
step, develop the different tissues and organs of the body.

By the end of thirty days the little embryo, about as large as one
inch of the end of a lead pencil, would be recognized as the embryo of
some mammalian animal, but it would be quite impossible to say whether
it would develop into a human being or some other animal, if it were
seen quite apart from its immediate surroundings. By the end of
another thirty days, however, the little embryo has multiplied its
size several times, and has reached a form instantly recognizable as
the young of the human kind, as shown in Figure IV. It still, however,
retains the vestige of a little tail, which within the next thirty
days will have been