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Preimplantation Period - 1st week
**Takes place during the first week after conception
**Union of ovum & sperm form zygote
**Meiosis combines chromosomes from ovum & sperm, giving the zygote a mix of 46 chromosomes
**Mitosis causes the zygote to become a blastocyst
**The blastocyst becomes embedded in the uterus (implantation)
**Down syndrome can happen during this stage
**Ectopic pregnancy can happen during this stage -
2nd week - Embryonic Period
**Proliferation of embryonic cells cause the blastocyst to grow
**Differentiation results in changes in cellular morphogenesis
**Embryonic cell layers (germ layers) are formed
(ectoderm, mesoderm, endoderm)
**A bilaminar disc is developed from blastocyst and is suspended between the amniotic cavity and the yolk sac
**The placenta develops from interactions of trophoblast & endometrial tissue -
3rd week - Embryonic Period
**Ectoderm gives rise to the CNS
**Mesoderm gives rise to connective tissue such as bone, cartilage, blood and muscle
**Endoderm gives rise to respiratory epithelium and cells of glands
**The embryonic disc forms a cephalic end which houses the oropharyngeal membrane, which will become the mouth.
**The embryonic disc forms a caudal end which will become the anus -
4th week - Embryonic Period
**Embryonic folding takes place to form tissue types
**Folding causes future embryo to become more recognizable
**Future digestive tract separates into three, forgery, midgut and hindgut.
**The face and neck begin to form with primitive eyes, ears, nose, oral cavity and jaw.
**Tongue begins to form
**The mandibular arch is formed
**Frontonasal process forms
**Lens placodes, two otic placodes, and two nasal placodes form
**Maxillary process is formed -
5th week - Embryonic Period
**Heart and circulatory system form
**During this week a heartbeat can be heard via ultrasound
**The yolk sac is replaced by the umbilical cord -
6th week - Embryonic Period
**Induction stage begins for tooth development
**Kidneys, liver and lungs are developing
**Cheeks, jaws and chin begin to form
**Resembles a tadpole with a tail (which is the spine)
**Upper & lower lips start to form -
7th week - Embryonic Period
**Blood starts to move through vessels
**Arms and legs start to bud
**Brain develops
**Embryo develops into a C shape -
8th week - Embryonic Period
**Lymphatic system begins to form
**External genitalia begin to form
**Arms and legs are longer
**Hands have fingers but toes may still be webbed
**Most all processes that have begin in prior weeks, continue maturing and growing until about week 12
