Biology 3338A Lecture Notes - Lecture 12: Primitive Streak, Germinal Disc, Hypoblast

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Chick gastrulation is very different from frog gastrulation except identical dorsal organizer concept. Chick gastrulation is more similar to human gastrulation with primitive streak formation. Female chicken mates with male and can store sperm for few days. Slowly fertilizes eggs within and then lays them. When egg gets fertilized in oviduct, almost no albumen and no egg shell. After fertilization, egg travels down oviduct, layers of albumen and shell secreted to cover outside. Chicken eggs we buy are actually not fertilized. Discoidal = blastodisk containing little cytoplasm on top of yolk. Meroblastic = cleavage occurs in blastodisc only ie. free of yolk. Cleavage only occurs on 2-3mm blastodisc on yolk. Initially yolk + little cytoplasm on top = one cell. Then cleavage divides cytoplasm that is still connected to yolk. Eventually, cellularization to form single layer of cells separate from yolk that sits on top of yolk. Then disc cells start secreting albumen below their surface to form subgerminal cavity.

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