Development Chapter 29

Fertilization

Zygote undergoes a series of mitotic cell divisions called cleavage zygote  2-cell stage  4-cell stage  8-cell stage  morula  blastocyst  gastrula Morula = solid sphere of cells (day 5) Blastocyst = hollow ball of cells (days 6-7), it produces human chorionic gonadotropin (hCG) hormone which can be detected in the woman’s urine during a pregnancy test; source of embryonic stem cells Gastrula = differentiation starts; contains three germ layers (ectoderm, endoderm, mesoderm; days 7-10)

Late Gastrula Mesoderm Endoderm Ectoderm

Three germ layers in gastrula: Ectoderm – gives rise to the nervous system and skin Endoderm – gives rise to lining of the gut and internal organs Mesoderm – gives rise to skeleton, muscles, circulatory system, reproductive system

Pregnancy becomes established during days : Fluid filled amniotic cavity starts to form Yolk sac starts to form (will make blood cells, germ cells) Embryo starts to form from embryonic disc Chorion (placenta) starts to form and will supply the embryo with nutrients and eliminate wastes Woman misses period

Development of the Placenta

Embryo = the term used until the 8 th week; from Greek “swelling within” Fetus = the term used from the 8 th week until birth; from Greek “offspring” Pregnancy can be divided into three trimesters: First trimester = body systems form (during this time the embryo is extremely sensitive to smoking & drugs) Second & third trimesters = continued body growth

BIRTH 3 parts: 1.Dilation of cervix 2.Expulsion of the baby 3.Expulsion of the placenta