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- the inital period of embryological and fetal development
- by the end of this first three months all of the organs systems have rudimentarliy developed.
- four major events occur during the first trimester
- when the secondary oocyte is fertilized it becomes a single cell that is acalled the zygote
- the zygote then undergosed a series of subdivisions
- these subdividons reduce the amount of cytoplasm in the cells, which are called blastomers
subdivide for about three days to form a solild ball of celles called the morula
reaches the uterus about day four and during the next two days it develops inot a hollow ball of cells called the blastocyst
- is composed of two cell layers
- the outher layer develops into the placenta
- the cells of the inner layer are clustered at on end of the blastocyst. and eventually develop inot the embryo
- occurs when the blastocyst becomes embedded in the endometrium of the mothers uterus.
- the outer cell layer becomes intimately associated with the maternal circulatory system for nutrient and gas exchange.
- meanwhile the inner cell layer of the blastocyst also differentiates
- the inner cell lay becomes detached form the outer cell layer
- the separtation yeilds a fliuid filled cavity called the amniotic cavity
- by the twelfth day a third cell layer develops and the blasotcyst become a struture called the gastrula
- during gaturaltion three germ layers form that five ries to the various organs systems
- these three germ layers form a sheet of cells called the embryonic disk
- and these three germ layeres then develop into the four extaembyonic membranes
- and these membrans protect the embryo
- this sac is a pouch formed from the endodem and mesderm.
- it is an intital site of blood cell formation
- this membrane is formed from the ectoderm and mesoderm
- it lilnes the innersurface of the amniotic cavity
- the fluid contained within the cavity is called the amniotic fluid
- the amnion and the amniotic fluid surround and protect the growing embryo
- this membrane is forme from the endoderm and mesoderm
- it eventually gives rise to the urinary bladder
- this membrane is formed from the mesoderm and the outer cell layer of the blastocyst
- the chorion differentiates into the featl portion of the placenta as the embryo enlarges
is the development and differentiaiton of the featal and maternal portions of the placenta
- fingerlike projections that chorion devlopes
- which make contact with the endometruim
- as the placenta enlarges it creates a large bulge on the surface of the endometrium
- blood ruputured maternal blood vessels slowly leaks through small chambers in the materanl placenta called lacunae
- the differentation of the nbody of the embryo and the rudiimentary development of the organ system
- the body of the embryo begins to emereg from the embryonic disk, and devlops into a bulge that has head and tail folds
second and third trimesters
- the fetus gains weight and the organ systems continue to devleop
- by the end of the pregnacy th uteus with the fetus occupy a large portion of the materanl abdominal cavity. s
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