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Natural Science Lecture # 17 Power Point Notes .docx

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Natural Science
NATS 1675
Barbara Czaban

November 15, 2012 Natural Science – Human Development Lecture # 16 PowerPoint Notes From Egg to The Living Body 2. The Pre-Embryonic Period: Cleavage and Implantation References: Custom Textbook  An Introduction to the Biology of Human Development, 2012 - Chapter 17, Development and Aging, pages 185 – 208
 - Chapter 20 (Sherwood), pages 160-168 Cleavage partitions the zygote into a ball of many smaller cells  Each new cell contains - Different regions of the original cytoplasm - Thus having different cytoplasmic determinants - Leading to different developmental fates for different cells Dividing fertilized sea urchin cell st  On 1 day, fertilization occurred in the upper third of oviduct  On 2 day (approx.), single-celled zygote undergoes cleavage – Mitotic divisions - - Cleavage continues to rapidly as the pre-embryo moves through rest of oviduct toward uterus - Sometimes mass of cells splits into 2 Cleavage in a frog embryo o Monozygotic twins – identical twins – from 1 zygote Genetically identical – same gender o Incomplete splitting results in conjoined twins - Fraternal twins occur when two secondary oocytes are released during ovulation – fertilized by two separate sperm o Dizygotic twins – non-identical twins – from 2 zygotes Not genetically identical – may be different gender  By about 4 day - The pre-embryo reaches the uterus - Is as the morula stage – a solid ball of 12 or more cells - Cells are small and tightly bound within the zone pellucida - Morula is about same size as zygote wa
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