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Travels of a Young Zygote

Travels of a Young Zygote - Stages The period of time from...

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Travels of a Young Zygote Cleavage of the zygote begins while it is still in the oviduct, producing a solid ball of cells (morula ). The morula enters the uterus, continuing to divide and becomes a blastocyst . Implantation The uterine lining becomes enlarged and prepared for implantation of the embryo in the trophoblast layer. Twelve days after fertilization, the trophoblast has formed a two- layered chorion . Human chorionic gonadotropin (hCG) is secreted by the chorion, and prolongs the life of the corpus luteum until the placenta begins to secrete estrogen and progesterone. Home pregnancy tests work by detecting elevated hCG levels in the woman's urine. Your Placenta or Mine? Maternal and embryonic structures interlock to form the placenta, the nourishing boundary between the mother's and embryo's systems. The umbilical cord extends from the placenta to the embryo, and transports food to and wastes from the embryo.
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Unformatted text preview: Stages The period of time from fertilization to birth (usually 9 months) is divided into trimesters, each about three months long. During pregnancy the zygote undergoes 40 to 44 rounds of mitosis, producing an infant containing trillions of specialized cells organized into tissues and organs. The First Trimester The three embryonic tissue layers form. Cellular differentiation begins to form organs during the third week. After one month the embryo is 5 mm long and composed mostly of paired somite segments. During the second month most of the major organ systems form, limb buds develop. The embryo becomes a fetus by the seventh week. Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination, forming testes in XY fetuses and ovaries in XX fetuses. External genitalia develop....
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