instances could be collected from the whole range of medical
literature.

Twins are most frequently born to parents whose ancestors have
established this tendency; the trait is usually inherited from the
mother's family, though occasionally it is passed on through the
father. Of course, that does not explain the cause of twins, which in
reality may result from either of two circumstances. More commonly
their genesis depends upon the ripening of two eggs at about the same
time and the fertilization of both by two different spermatozoa. The
children, in this instance known as double ovum twins, may be of the
same sex or not. On the other hand, single ovum, or identical, twins
are always of the same sex; this follows, since but one egg and but
one spermatozoon are here concerned. The incident permitting twins to
develop from a solitary ovum must occur soon after conception has
taken place. It will be remembered that the first step in the
development of the fertilized ovum consists in its dividing into two
cells. Ordinarily, both these take part in the development of one
embryo, but occasionally they separate and give rise to two.
Frequently, the presence of twins can be recognized during the latter
months of pregnancy, and accurate means are known of determining
after they are born to which variety any given pair belongs.

THE RATE OF GROWTH.--When we recall the definite and often marked
differences in the physical character of women, such as weight and
height, it is surprising to learn that the prenatal development of
their children proceeds with uniform speed. One very practical result
is that the physician is thus enabled, at the birth of a premature
infant, to estimate accurately the period of its development. Various
criteria, some of which are easy of application, aid in this