Section 3: Pregnancy, Development, and Birth What are the changes that occur to the zygote, embryo, and fetus during development? How is the developing embryo protected and nourished? What happens during childbirth? What changes occur as a person develops from infancy to adulthood?

Development Before Birth The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs. Zygote

Development Before Birth The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs. Zygote Eight-Cell Stage Hollow Ball

Development Before Birth The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs. Zygote Eight-Cell Stage Hollow Ball

Development Before Birth The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs. Zygote Eight-Cell Stage Hollow Ball Stage When Folding Begins

Development Before Birth The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs. Zygote Eight-Cell Stage Hollow Ball Stage When Folding Begins Embryo With Three Layers

Protection and Nourishment The membranes and other structures that form during development protect and nourish the developing embryo, and later the fetus.

Birth There are two types of twins: identical twins and fraternal twins.