Embryo 51 Day Old Liver and Nervous System: Computer Generated Image from Micro-MRI, actual size of embryo = 18.0 mm - This image presents a left-sided view of the embryo during the eighth week of embryonic development, the age being calculated from the day of fertilization. The three major, differentiated components of the brain can be seen, with the forebrain protruding downwards, the midbrain as the narrow part of the brain, and the hindbrain that connects with the spinal cord. The light pink ring-like structure in the facial region is the developing eye, the rings highlighting eyelid formation. The larger groove with a black hole in the middle represents the growing external ear, the ring-like appearance indicating the auricle of the external ear. The hand plate has a web-like appearance as the digital rays slowly become more distinguishable from one another. The foot plate has the digital rays, but remains less distinguished than the hand plate. It typically develops a few days after the hand plate. The large red organ protruding is the liver. As well, the red tube-like structure near the foot of the embryo indicates the umbilical cord, which provides a means of transporting nutrients and wastes between mother and embryo.
Prenatal development is the process in which an embryo and later fetus develops during gestation. Prenatal development starts with fertilization the first stage in embryogenesis which continues in fetal development until birth.
In human pregnancy, prenatal development, also known as antenatal development, is the development of the embryo following fertilization, and continued as fetal development. By the end of the tenth week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.
In other animals the very early stages of embryogenesis are the same as those in humans. In later stages, development across all taxa of animals and the length of gestation vary.
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