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.
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.)
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