Early Development of the Embryo
The human embryonic period begins with fertilization and encompasses development through the first two months. During this time, the embryo's nascent organs and tissues are established, embryonic membranes develop, and the placenta begins functioning. After the first eight weeks of development, the embryo is called a fetus.
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Zygote
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A fertilized egg is called a zygote. At this stage it consists of a nucleus, cytoplasm (all parts other than the nucleus) and an external membrane. The cells divide rapidly, becoming progressively smaller as their numbers grow. A few days after fertilization, the mass of cells is called a morula, which moves down the fallopian tube and enters the uterus. By this time, what was once a dense cluster of cells has developed into a hollow ball with the cells forming an external layer. It is now called a blastocyst.
Blastocyst
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To envision the blastocyst, think about a round eye with a pupil. One part of the blastocyst is the inner cell mass, which is located in the blastocyst like the pupil is in the eye. The remainder of the inside part of the blastocyst is a fluid-filled cavity called a blastocele. There is also an outer covering called the trophectoderm. The blastocyst attaches to the endometrium---or lining of the uterus---and absorbs nutrients until the placenta forms.
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Organs and Tissues
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After implantation, the inner cell mass moves downward to create a space called the amniotic cavity. A layer of cells grows over the top of the cavity to create the amnion. The cells below the cavity form a flattened "plate" of cells called the embryonic disc. Around day 14, the cells in the embryonic disc develop into three distinct layers. The outer layer becomes the ectoderm, the middle layer the mesoderm, and the inner layer the endoderm. These are the three primordial cell layers that will develop into distinct functions.
The two ends of the embryonic disc close together to create a cavity in the center. This endoderm-lined cavity is the origin of all parts of the digestive tract, lungs, urinary bladder and thyroid gland. The mesoderm separates into two layers and the space between the two layers becomes the body cavity. The mesoderm forms the peritoneum, muscle, bone, connective tissue, blood and lymph vessels, kidneys, ureters and adrenal cortex. The ectoderm provides the cells that develop into the skin, eye, mouth, teeth, receptor cells of senses and the nervous system.
Embryonic Membranes
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Two membranes surround the embryo. The outer membrane, the chorion, becomes part of the placenta. The inner membrane, the amnion, fills with amniotic fluid to cushion and protect the developing embryo. A third membrane, the allantois, contains the supply of blood vessels that becomes the umbilical cord.
Placenta
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Projections called chorionic villi grow out of the chorion into a layer of the endometrium called the decidua basalis. Together they form the placenta. The blood of the mother and embryo do not mix, but the placental membranes are thin enough that nutrition and oxygen can pass from the mother's blood into the chorionic capillaries. Waste from the embryo transfers from the chorionic villi into the mother's blood. In addition to providing nutrition, the placenta also secretes the hormones that are necessary to maintain the pregnancy. By now the second month has ended and the embryo enters the next stage, fetal growth.
Overall Embryonic Development
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By the end of the first month the backbone and vertebral canal have formed. Small buds that will become the arms and legs are evident. Three major divisions of the brain are distinguishable and the heart has begun to develop. By the end of the second month, the eyes are noticeable but the eyelids are closed. Major blood vessels form and internal organs continue to develop. The limbs become distinct arms and legs, the digits are formed and the process of bone formation begins.
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