single cavity, and usually develops but a single embryo.
TWINS
Sometimes two ovules are matured at the same time. If fecundated, two
embryos instead of one will develop, producing twins. Triplets and
quadruplets, the results of the maturing of three or four ovules at
the same time, occur more rarely. As many as five children have been
born alive at a single birth, but have seldom lived for more than a
few minutes.
GESTATION
The development of the ovule in the womb is known as gestation or
pregnancy. The process is one of continued cell division and growth,
and while it goes on the ovule sticks to the inner wall of the womb.
There it is soon enveloped by a mucous membrane, which grows around it
and incloses it.
THE EMBRYO
The _Primitive Trace_, a delicate straight line appearing on the
surface of the growing layer of cells is the base of the embryonic
spinal column. Around this the whole embryo develops in an intricate
process of cell division and duplication. One end of the Primitive
Trace becomes the head, the other the tail, for every human being has
a tail at this stage of his existence. The neck is marked by a slight
depression; the body by a swollen center. Soon little buds or "pads"
appear in the proper positions. These represent arms and legs, whose
ends, finally, split up into fingers and toes. The embryonic human
being has been steadily increasing in size, meanwhile. By the fifth
week the heart and lungs are present in a rudimentary form, and ears
and face are distinctly outlined. During the seventh week the kidneys
are formed, and a little later the genital organs. At two months,
though sex is not determined as yet, eyes and nose are visible, the
mouth is gaping, and the skin can be distinguished. At ten weeks the
sexual organs form more definitely, and in the third month sex can be
definitely determined.
THE FOETUS