Image caption : Human Embryo 26 Day Old with Placenta ; Computer Generated Image from Micro-MRI, actual size of embryo = 4.0 mm - This image provides a left-sided view of the embryo during its fourth week of development. The age is calculated from the day of fertilization. The developing spinal cord can be seen, highlighted in dark yellow. The indentations in head region are pharyngeal arches, which contribute to formation and develop of the head and neck regions. The developing heart is highlighted in red, the left atrium can be observed. Early growth of the cardiovascular system begins during the third week, when blood vessels form, and continue into the following weeks of development. The round, red structure beside the embryo is the placenta. The placenta with the umbilical cord functions as a mean for transporting nutrients, waste products, and gases between mother and embryo.
Human embryogenesis is the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. Embryogenesis covers the first eight weeks of development and at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.
The germinal stage, refers to the time from fertilization, through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.
During this stage, the zygote, which is defined as an embryo because it contains a full complement of genetic material, begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow. The embryo is referred to as a fetus in the later stages of prenatal development, usually taken to be at the beginning of the ninth week. In comparison to the embryo, the fetus has more recognizable external features, and a more complete set of developing organs. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.
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