After
implanting, the embryo divides into three layers. The outer layer of cells
in called the ectoderm, and will develop into the outer layer of the skin
and the nervous system. The middle cells, or mesoderm, develops blood,
bone, cartilage, and muscle. The endoderm, the inner layer, develops
eventually into mucus membranes and glands. (7.)
The blood vessels in the embryo that link it to the placenta are one of the first structures to form in the embryo.Outside of the embryo, the yolk sac is formed and nourishes the embryo until the placenta is completed. (7.) In the chorion, the first blood cells and more blood vessels are formed. (13.)
In
the third week of development, or at two gestational weeks of age, cells
from the ectoderm fold to form the embryo's first primitive organ, the
notochord. (9.) The notochord is visible
in the picture above left, as a thin white streak in the center of the
disk-like embryo. Amazingly, this tiny, simple structure is the beginning
of the human brain.
The embryo's heart is also developed in the third week. (13.) By the end of the week, just 21 days after this creature came into existance as a single fertilized ovum, the heart has begun to beat. It will continue to beat throughout the individual's life.
The
fourth week is one of amazing developments. The notochord fuses into the
neural tube which, by the end of the week, has folded over and bulged at
one end to form the brain. (
Nearly all the embryo's other organ systems have their beginning in the
fourth week of development. 
Circulation
between the embryo and the placenta begins, blood pumped by the
one-chambered embryonic heart. (13.)
From the neural tissues, the eyes and ears begin to form. (7.)
The tongue, esophagus and stomach are well developed, and the beginnings
of the liver, gall bladder, and pancreas appear. Lungs begin to develop,
as do kidneys, and the thyroid gland. Towards the end of the week, limb
buds appear. (13.)
The
embryo's development continues rapidly in the fifth week. The brain
divides further. The forebrain develops into two distinct sections, called
the telencephalon and the diencephalon. The telecephalon is the primitive
cerebrum, the 'thinking' part of the brain. The cerebrum is responsible
for sensory perception, memory, learning, and conscious thought. The
diecephalon develops into the thalumus and the hypothalumus, the 'feeling'
part of the brain. The thalamus serves as a relay station between the
senses and the brain. The hypothalamus produces basic drives and emotions
such as hunger, thirst, pleasure, and fear. The midbrain continues to
develop into brain structures also necessary for processing sensory
information, while the hindbrain grows towards becoming the cerebellum,
medula, and pons. These parts of the brain are responsible for unconscious
physical processes like blood circulation or breathing, as well as
reflexes. Also, the structures of the hindbrain are necessary for muscle
coordination and movement. (12.) The
well-developed brain is visible in the picture above left, as are the
embryo's developing hands.
The limb buds develop into more clearly differentiated limbs and feet or
hands in the fifth week. Facial features begin to develop, most especially
the nose, which was previously absent. The picture at left shows the
embryo's developing face. Also in the fifth week,the kidneys develop a
great deal, the stomach assumes its adult position, and the embryo's sex
organs begin to develop. (13.)
In
the sixth week, the heart develops four chambers, the eye grows more
complex, salivary and adrenal glands appear. The palate is formed, with
tooth buds. The sex organs develop further; it is now possible to tell if
the embryo is a boy or girl. The face's development is complete.(13.)
Though the embryo won't have the muscles to smile or frown for several
months yet, she looks essentially like a baby in minature now. Her head is
still very much out of proportion to her body, visual evidence of the
importance of the brain in comparison to the rest of the organs. At 40
days, the embryo's brain waves can be detected by an electroencephalogram.
(8.)
The most striking development of the sixth week is not in the structure of the embryo, but in newly formed ability. At six weeks, she has simple reflexes (13.)and she begins to move. (7.) The mother won't feel these motions for another several months, since her child is still far too small to actually stretch the uterus with her motion, but these first movement are important to the healthy development of the muscles. (5.)
It is interesting to note that at six weeks, the structures in the brain that control movement in adults are not close to fully developed or even differentiated. Yet, the embryo moves. Despite the absense of myelin, a substance that coats part of nerve cells and is absolutely crucial to the transmission of neural impulses in born individuals, (12.) the embryo's nerves are obviously working. The embryo is doing things that an adult with similar brain development would be completely incapable of doing. Just as the heart, which beat first with one chamber, the brain, nerves, and muscles are obviously working far before completion. This has interesting implications in terms of the embryo's mental abilities. Can the embryo think, with it's partially developed cerebral cortex, or feel with the midbrain only partially differentiated? Perhaps yes, perhaps no, but many facts seem to suggest that the body in general begins to function long before it is fully formed. Due to the lack of myelin, many scientists assume that an unborn baby can't think or feel until well into the third trimester, but there is much evidence to suggest otherwise. It seems reasonable to assume that consciousness and thought develop apace with everything else. In the six week old embryo consciousness, like movement, is probably primative, but it seems quite likely that it is there.
In
the seventh or eighth week, the embryo becomes sensitive to touch in parts
of the body. If the embryo's cheek is stroked, she will arch her back and
throw back her arms. (11.) Sensation
spreads slowly over the rest of the body as development progesses and her
neural networking becomes more complex. (3.)
The embryonic period ends at the end of the seventh week gestation. At this point, all the embryo's major organ systems are at least begun. (7.) Many of them, most notably the circulatory system, are functioning. (13.) She looks like a miniature baby, with well defined fingers and facial features. As the embryo becomes a fetus, the main focus of her development shifts from the genesis of new organs and organ systems to growth and refinement of already existing systems. (13.)