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The first stage of human development begins with fertilization of the egg by a sperm. This act forms the first cell known as the zygote.
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In the next few days the cell divides. It divides so fast that each individual cell only divides and does not grow. It eventually becomes a solid mass of cells, called a morula.
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The mass of cells has now developed enough that it is a blastocysts. After the zona pellucida and trophoblast deteriorate from the blastocysts, it can attach itself to the uterine wall.
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The blastocyst begins implantation in the epithelial lining and underlying stroma. The cellular product is now referred to as the conceptus. When the conceptus implantation is done, a coagulation plug is formed and the entire process of development will happen within the wall of the uterus.
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The conceptus completes implantation. The trophoblast cells multiple rapidly. The endometrium around the cell begins the decidual process and lines the conceptus. The extra-embryonic cavity starts to form. The bilaminar embryonic disc forms from the inner cell mass.
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The amniotic, primitive yolk sac, and chorionic embryonic spaces are now present. Chorionic villi are beginning to from the chorionic cavity. These villi are the beginning of the functional units of the placenta.
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Cells migrate from the epiblast and the mesoderm starts to form and grow. The mesoderm lies between the endoderm and ectoderm. A tube extends from the primitive node under the ectoderm in the opposite direction to the primitive streak. This tube forms the first process of the notochordal and notochord. The notochord is important in the folding and regulation of the endoderm and ectoderm. 1-3 somite pairs begin to appear during the end of this stage.
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Somites 4-12 begin to form. These include things like neural folds in the brain, neural tubes, caudal neuropore, neural fold fusses and the remnant of amniotic sac. Next features of the heart tube neural plate to tube begin to develop.
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Neural tube continues to close, and the caudal neuropore closes. The mesoderm continues to grow with heart prominence. Different structures in the head begin to form. The heart, liver, umbilical cord, and upper limbs all begin to form as well.
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The endoderm and mesoderm continue to form. The head continues to form like the forebrain, site of lens placode and the stomodeum. The body organs and upper and lower limbs continue to form as well.
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The ectoderm and mesoderm continue to form. The head including the forebrain, eyes, and auricular hillocks. The body continues to grow including the organs in it like the heart and liver. The lower and upper limbs also continue to grow.
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The ectoderm like sensory placodes, lens pits and otocysts are continuing to form. As well as the mesoderm like heart prominence, and the head as in the forebrain and eyes. Limbs of the body are still growing. Knees and elbows are becoming more prominent. Hearing also starts to develop during these stages also.
