(after conception and implantation)
|Remember that fertilization happens in the fallopian tubes . . . implantation happens 7 to 9 days after fertilization in the UTERUS (ideal conditions) . . . at 4 days
following fertilization, that little mass of cells is called a BLASTOCYST.
HUMAN BLASTOCYST, and you don't have to remember the parts listed.
The one-celled zygote created at conception divides by mitosis into 2 genetically identical cells within 12 to 15 hours... and then
another division to make 4 cells within the next 12-15 hours... and so forth. By the third day following conception the cells have
divided to form a small ball that is known as a blastocyst. The blastocyst enters the uterus and begins to implant in the blood rich
lining of the uterus.
Implantation occurs when the blastocyst sinks into the endometrium. Through the action of hormones produced by the blastocyst,
an intensive chemical exchange occurs between the blastocyst and the mother's tissue to signal its presence in the uterus. This
exchange temporarily impairs the mother's immune system to prevent antibodies from attacking the blastocyst as foreign matter.
During the second week after conception, the inner cell mass of the blastocyst structure separates into three distinct layers that
may be seen under a microscope. The outer layer is known as the ectoderm, which will later form the skin, hair, sweat glands,
tooth enamel, salivary glands, and all the nervous tissue, including the brain. The middle layer, or mesoderm, will form the
muscles, bones, blood, circulatory system, teeth, connective tissues and kidneys. The inner layer, the endoderm, will form most
of the internal organs (stomach, intestines, liver, lungs, heart, and so on).
These three layers are differentiated by the third week following conception. They will continue to proliferate into the various
organ system tissues during the embryonic period of prenatal development.
|4 week old embryo
|An EMBRYO. The embryo is the name of the prenatal organism (baby) from 2 to 8 weeks after conception, during which time the foundations of all body structures and organs
are laid down.
By four weeks, the embryo has a head, tail, backbone and limb buds - which will eventually become arms and legs. The beginnings of ears and eyes are also visible. Its heart
is already beating (starting at day 21), and the other organs are forming fast. An umbilical cord starts to grow between the embryo and the placenta. During this time, the
embryo is especially sensitive to any drugs or infections capable of crossing the placenta from the mother.
By the end of week 8 (the 10th week of pregnancy), all the different parts of the body are in place. Fingers and toes
form - the embryo now has unique fingerprints. Its head is still very large compared to its body - almost half its length. The
brain is growing at about 100,000 new brain cells every minute. All embryos look identical at seven weeks, before the sex-
determining gene is switched on in males.
From week 9 (the 11th week of pregnancy) the developing embryo is called a FETUS. The size of its body catches up
with the outsized head, making it look more human, although the legs are still slightly short. Protected by the amniotic fluid,
the fetus can move around and flex its limbs. By 12 weeks, it can hear sounds, and its skin is sensitive to touch.
The fetus grows very quickly during weeks 13 to 16 - doubling in size from 5 to 10 centimeters. The face starts to form,
and by 14 weeks (the 16th week of pregnancy) the eyes can move. The eyelids stay closed until the final two months
of pregnancy. Now that the internal organs are in their final positions, the bones begin to harden, although they remain
flexible until after the baby has been born.
By 20 weeks (the 22nd week of pregnancy), the mother can feel the fetus move. It has eyebrows and hair on its head.
Its entire body is covered in fine, downy hair. A greasy substance called vernix protects the skin. The fetus practices
swallowing and digesting fluid, and can even tell sweet tastes from bitter ones. An ultrasound scan may now reveal
whether the fetus is a boy or a girl.
During the second half of pregnancy, the fetus becomes increasingly aware of the world outside. It is startled by
sudden noises, and is thought to be capable of feeling pain after 5-6 months. Rapid eye movements, associated with
dreaming sleep in adults, begin at 21 weeks (the 23rd week of pregnancy). The fetus looks transparent, as it doesn't
yet have any fat under its skin.
By 26 weeks (the 28th week of pregnancy), the lungs are ready to breathe air. The brain has developed enough to
control breathing and body temperature to some extent. So from now on the fetus stands a good chance of surviving even
if it's born early. It has eyelashes and fingernails. It appears less wrinkled, as more and more fat is laid down under the
skin. The baby's skin is now pink and smooth, and its arms and legs are fatter. The eyes have opened, and by 30 weeks
(the 32nd week of pregnancy) the pupils will dilate and contract in response to light. The mother may feel the baby
hiccuping, after it has swallowed too much amniotic fluid. By 34 weeks, most babies have their heads downwards, ready
for birth. The baby continues to put on weight, reaching an average of 3.4 kilograms by 38 weeks (the 40th week of
pregnancy). It is outgrowing the womb, and starts the birth off by releasing hormones. The muscles of the mother's womb
start to contract, and labor begins. Most babies are born within ten days of their due date - but only 5% actually arrive on
the predicted day.
|The fetus at 14 weeks, National Medical Slide Bank/Wellcome Photo
Below, An ultrasound scan of a 22-week-old fetus.
|Electron micrograph of a four-day-old human embryo.
Yorgos Nikas/Wellcome Photo
|The hands and mouth of a fetus around six months
Petit Format, Nestle/Science Photo Library