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2. PRENATAL PERIOD
Three primary germ layers appear in the developing embryo after implantation of the blastocyst (Table 21-1):
Endoderm—inside layer
Mesoderm—middle layer
Ectoderm—outside layer
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3. PRENATAL PERIOD Histogenesis and organogenesis
Formation of new organs and tissues occurs from specific development of the primary germ layers
Each primary germ layer gives rise to definite structures such as the skin and muscles
Growth processes include cell differentiation, multiplication, growth, and rearrangement
From 4 months of gestation until delivery, the development of the baby is mainly a matter of growth
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4. PRENATAL PERIOD Birth defects
Any structural or functional abnormality present at birth
May be caused by genetic factors
Abnormal genes
Abnormal number of chromosomes
May be caused by environmental factors
Environmental factors are called teratogens
Include radiation, chemicals, and infections
Especially harmful during the first trimester (Figure 21-7)
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6. BIRTH, OR PARTURITION
Process of birth called parturition (Figure 21-8)
At the end of week 39 of gestation, the uterus becomes “irritable”
Fetus takes head-down position against the cervix
Muscular contractions begin, and labor is initiated
Amniotic sac (“bag of waters”) ruptures
Cervix dilates
Fetus moves through vagina to exterior
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