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Chapter 4

Chapter 4 - Textbook Notes

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Western University
Psychology 2040A/B
Ian Mac Donald

CHAPTER 4 Prenatal Development development that occurs between the moment of conception and the beginning of the birth process FROM CONCEPTION TO BIRTHDevelopment begins in the fallopian tube when a sperm penetrates the wall of an ovum forming a zygote From the moment of conception it will take approximately 266 days for this zygote to become a fetus that is ready to be born often divided into 3 major phasesPrenatal developmento Phase 1Period of the Zygote Germinal Period lasting from conception until the developing organism becomes firmly attached to the wall of the uterus Normally lasts about 1014 daysrdtho Phase 2Period of the Embryo lasting from the 3 week through the 8 prenatal week during which the major organs and anatomical structures take shapetho Phase 3Period of the Fetus lasting from the 9 prenatal week until birth during this period all major organ systems begin to function and the fetus grows rapidly The Period of the ZygoteAs the zygote moves down the fallopian tube toward the uterus it divides by mitosis into 2 cells These 2 cells and all the resulting cells continue to divide forming a ballshaped structure or blastocyst that will contain 6080 cells within 4 days of conception Cell differentiation has already begun Inner layer of the blastocyst becomes the embryoOuter layer of cells will develop into tissues that protectnourish the embryoImplantation Blastocyst approaches the uterus 610 days after conceptionsmall burrlike tendrils emerge from its outer surface Upon reaching uterine wall these tendrils burrow inward tapping the mothers blood supplyImplantationo Specific window of implantation during which the blastocyst must communicate with the uterine wall position itself attach and invade Implantation takes about 48 hoursOccurs 710 days after ovulation Entire process of implantation is completed 1014 days after ovulationNearly 3 out of every 4 zygotes fail to survive the initial phase of prenatal development Development of Support SystemsOnce implanted the blastocysts outer layer rapidly forms 4 major support structures that protectnourish the developing organism 1 Amnion watertight membrane that surrounds the developing embryo serving to regulate its temperature and to cushion it against injurieso Fills with fluid from the mothers tissuesamniotic fluido Balloonshaped yolk sac floats in this watery environment to produce blood cells for the embryo until it is capable of producing them on its own 2 Chorion membrane that becomes attached to the uterine tissues to gather nourishment for the embryoo Yolk sac attaches to the chorion 3 Placenta an organ formed from the lining of the uterus and the chorion that provides for respiration and nourishment of the unborn child and the elimination of its metabolic wastes 4 Allantoisforms the embryos umbilical cord a soft tube containing blood vessels that connects the embryo to the placentaPurpose of the PlacentaOnce developed the placenta is fed by blood vessels from the mother and the embryoo Hairlike villi act as a barrier that prevents these two blood streams from mixingo This barrier is semipermeableallows some substances to pass through blood vessels are too big Maternal blood flowing into the placenta delivers oxygen and nutrients into the embryos bloodstream by means of the umbilical cord which connects the embryo to the placenta Umbilical cord also transports carbon dioxide and metabolic wastes from the embryocross the placental barrier enter the mothers bloodstream and are expelled from the mothers body along with her own metabolic wastes Placenta is the site of all metabolic transactions that sustain the embryoThe Period of the Embryordth Lasts from implantation 3 week to the 8 week of pregnancyrd By the 3 week the embryonic disk is rapidly differentiating into 3 cell layers o Ectoderm outer layerwill become the nervous system skin and hair o Mesoderm middle layerwill become the muscles bones and circulatory systemo Endoderm inner layerwill become the digestive system lungs urinary tract and other vital organs like the pancreas and liverrd In the 3 week a portion of the ectoderm folds into a neural tube that soon becomes the brain and spinal cord becomes the central nervous systemth By the end of the 4 week the heart has formed and begun to beato Eyes ears nose mouth beginning to formo Buds that will become arms and legs appear At no time in the future will this organism ever grow as rapidly or change as much as it has during the first prenatal month
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