Instructions

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The Zygote A fertilized egg	Leave this space blank	2 cell stage Cells have divided once. Notice, the size is the same as the zygote
4 cell stage Again, number of cells increased but size didn't	morula A solid ball of 16 cells. It is still the same size as the zygote	Blastula A hollow ball of cells formed from after the morula has divided a few times. Note the embryonic disc (yellow). Also note, it is the same size as the zygote.
	Gastrula The embryonic disc becomes a multi layred structure and it will start to invaginate on one end becoming a gastrula. This will result in 3 germ layers around the gut formed by the gastrula. The inner most germ layer surrounding the gut is called the endoderm. The endoderm will develop into the respiratory and digestive systems. The outer layer is called the ectoderm and it forms the skin and the nervous system. The third layer, found between the other two, is the mesoderm forms the circulatory urinary, and musculoskeletal systems.
	Embryo Defined as weeks 3-8, organs systems start to develope. The digestive system forms around the endoderm while the mesoderm forms the circulatory and musculoskeletal systems. More importantly, the mesoderm forms the notocord wich will guide the development of the nervous system from the ectoderm. The yolk sac forms around the embryo and produces blood cells while the amnion surrounds the embryo produces amniotic fluid which provides a stable environment for the embryo and fetus to develop. A chorion also develops which will form the placenta
	The Fetus By the end of week, most internal organs have formed and developing baby is now considered a fetus The fetus will have recognizable limbs and a defined head. A well-defined placenta has also formed. For the next 28-32 weeks, the fetus will continue to grow and the organs will develop.