Fertilization and Development Gestation

Fertilization Internal Fertilization – Mammals, Birds, Reptiles  Small # 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