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Fertilization• Internal Fertilization– Mammals, Birds,
ReptilesSmall # of offspring
• External Fertilization– Fish, Amphibians
Large #’s of offspring
Internal vs. External Development
Internal • Offspring develop inside of
the mother’s uterus• Small # of offspring• Offspring are protected
during development• Mammals - mainly land
dwelling but whales and dolphins are exceptions
External• Offspring develop outside of
the female’s body, usually in water
• Large # of offspring• Offspring are not well
protected• Birds, reptiles, amphibians
and fish
Early Stages of Development• Zygote – fertilized
egg• Morula – solid ball of
cells• Blastula – larger
hollow sphere of cells• Gastrula – infolding
of the blastula forms layers
Layers of the Gastrula
• Endoderm– Inner layer
Forms organs spinal cord and brain
• Mesoderm– Middle layer
Forms muscles and bones
• Ectoderm– Outer layer
Forms skin
Placenta
• Delivers oxygen, water, sugar and nutrients to the fetus
• Removes carbon dioxide, urea and other waste products produced by the fetus