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Cleavage - Bi Sci 001 Monday Announcements Review Session...

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Bi Sci 001 Monday March 29, 1999 Announcements: Review Session tonight Lecture notes: Cleavage Division of the fertilized egg into a ball of undifferentiated cells. Day 1 2 cells Day 2 2 cells --- 4 cells Traveling through the oviduct Day 3 4 cells --- 8 cells Day 4 8 cells --- 16 cells ---- This clump of cells is called a morula. Day 4-7 ?? Arrival at uterus. It has now become a hollow ball of cells called a blastocyst. Implantation At about 1 week, the blastocyst burrows into the endometrium (inner layer of the uterus). The process takes about 1 week. Some of the outer layer of the blastocyst forms the chorion which aids in implantation. Gastrulation Migration of cells in the blastocyst (blastula) to form three layers. It occurs during the week 2 of pregnancy. Extremely important step in development. (Some say most important) One third of all embryos either don't "do" gastrulation of don't do it correctly. (This results in the loss of the embryo in most cases) Siamese twins are a result of "double gastrulation".
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