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We know from the science of embryology that within the first seventeen days of the embryonic stage, three germ layers (Endoderm, Mesoderm and Ectoderm) develop from which all the organs and tissues originate. When a fetus develops during the embryonic phase (ontogenesis), the growing organism passes in a highly accelerated speed through all the evolutionary stages (phylogenesis). During this journey through the stages of evolution - from a single celled creature to the complete human being - the three germ layers direct step by step the development of the entire organism.
Most of our organs, notably the colon, emanate from only one of the three germ layers; others such as the heart, the liver, the pancreas, or the bladder, are made up of different parts deriving from different germ layers (Metastasis Theory). Today these parts, which merged over time for functional reasons, are regarded as one organ, even though they often have their control centre in widely separated areas of the brain. On the other hand, there are organs that lie far apart from each other in the body such as the rectum, the larynx, and the coronary veins, but are controlled from areas that are close together in the brain.
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