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Cleavage
early, rapid cell division without growth that creates a hollow ball of cells (blastula)
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Gastrulation
- cells rearranged into 3-layered gastrula
- layers now in the ball of cells
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Organogenesis
layers interact and give rise to organs
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Acrosomes
hydraulic enzymes (enzymes that break things down with water)
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Fertilization envelope
- develops once sperm gets inside an egg
- then the whole chemical balance of the egg has been changed by the sperm (this is the reason only the first sperm can fertilize an egg)
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Proteins in sperm
match up with the receptor of the egg
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Why is there no growth in cleavages?
The cells divide so fast (as soon as they can), they basically skip G1
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Vegetal pole
contains lots of yolk
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Animal poles
contains little yolk
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Gastrulation
- rearranges the cells of a blastula into a 3-layered embryo, called a gastrula, which has a primitive gut
- goal: to give actual layers that wrap around each other; mass movement of cells
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Ectoderm
outer layer (skin, sweat glands, hair follicles, tooth enamel, etc.)
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Mesoderm
middle layer (skeletal, muscular, and reproductive systems, etc.)
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Endoderm
inner layer (liver, pancreas, linings of reproductive system, urethra, digestive system, etc.)
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Two general principles of the developmental fate of cells
- During early cleavage divisions, embryonic cells must become different from one another
- After cell asymmetries are set up, interactions among embryonic cells influence their fate
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3 axes
- Proximal-distal axis
- Anterior-posterior axis
- Dorsal-ventral axis
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