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The sperm fertilizes the egg to form a zygote in the falopian tube; begins 1st trimester
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A mitotic cell division occurs in the zygote multiple times as the zygote becomes a pre-embryo and develops into a multicelular blastocyst; eventually ends when uterine wall is contracted
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A solid ball of blastomere cells forms from cleavage phase; reaches uterus on 4th day
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The morula develops inner cavities and an inner cell mass; formed by many blastomeres as division continues
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The blastocyst adheres to the uterine lining as the trophoblast cells rapidly divide to create several layers
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The 3rd layer of cells begin to form (mesoderm) as others move towards the central line; the primitive streak begins to form
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The neural grooves and folds of the developing brain are first seen
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1-3 somite pairs appear in the neural groove and can now be added to sequence to develop the musculoskeletal system in the embryo
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Heart cardiogenesis occurs on day 21 as paired tubes; laryngotracheal groove (precursor to lungs) forms on day 22 to begin respiratory development; Thyroid development begins on day 24; Liver formation begins on day 25
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The hindbrain is supplied by 2 arteries to begin the development of the circulatory system and bloodflow
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Sensory receptors and sight and smell structures become prominent in the embryo while the 4th brain ventricle also appears; The heart, liver, and umbilical cord become externally visible as bulges protruding from the embryo
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The pituitary gland becomes defined and independent in the proper location; Parathyroid glands generate in the thyroid; Adrenal glands begin to form; Heart and lungs descend into thorax; tongue begins to become defined
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Olfactory nerve fibers enter the brain to provide the early parts of the smell sense; Eyelid sulcus deepen as eyelid folds develop
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The pancreas secretes insulin at an increasing rate; Liver enlargement stops the movement of the heart and lungs, keeping them at the desired location
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The abdominal cavity becomes defined to provide structure with the presence of muscle arising along with connective tissue presence; The gastrointestinal tract seperates from the anal membrane
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Testosterone production begins in the embryo
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Body trunk straightens; Rounded head; Abdominal wall forms 3 layers; G cells in stomach begin to appear; Fetal development begins
