Fertilization and Development

Gestation

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