Animal Reproduction and Development

• Gametes, or sperm and egg, are produced within the gonads, the testes and ovaries, of animals through meiosis.
• Fertilization is the process where an egg and sperm unite, resulting in a diploid zygote, which has two sets of chromosomes, one from each parent.
• In internal fertilization, eggs and sperm meet inside of one of the parents, usually female, and develop.
• In external fertilization, eggs and sperm are dispersed into the medium, usually air or water, in hopes that they meet and fuse.
• Gametes and offspring are highly vulnerable to environmental conditions during fertilization and embryonic development.
• The zygote grows through cellular division, giving rise to various diploid embryonic stages.
• Cleavage of the zygote exponentially increases numbers.
• A solid morula forms during embryonic development.
• The hollows out to become a blastula during embryonic development.
• Gastrulation occurs and cells migrate to form the three tissue layers, ectoderm, mesoderm, and endoderm, during embryonic development.
• Neurulation leads to the development of the nervous system, forming a neural tube that eventually becomes the brain and spinal cord, during embryonic development.
• Incomplete metamorphosis: Juvenile stages resemble adults, differing only in size and sexual maturity, is observed in reptiles and mammals.
• Complete metamorphosis: Organisms undergo dramatic transformations in form and ecology as they move from the juvenile (larval) stages to adult stages, is observed in aquatic invertebrates, insects (caterpillars à butterflies), and amphibians (tadpoles à frogs).
• Embryonic development in sea stars involves one mitotic division, four rounds of mitosis, and two cleavage divisions.
• Embryonic development in frogs involves one mitotic division, four rounds of mitosis, and two cleavage divisions, and an additional process of neurulation between the gastrulation and organogenesis stages.
• During neurulation in frogs, the brain and spinal cord begin to develop, the ectoderm flattens, forming a neural plate, and the folds thicken and roll around the groove, forming a depression that will separate as the neural tube, the embryonic central nervous system.
• Observing Eggs, Sperm, and Fertilization in Sea Urchins involves watching male sperm compete with female eggs.