HiSET Science: Reproduction, Development, and Growth in Animals

Gamete Formation
Gametogenesis is the formation of gametes, or reproductive cells. Gametes are produced by meiosis. Meiosis is a special type of cell division that consists of two consecutive mitotic divisions referred to as meiosis I and meiosis II. Meiosis I is a reduction division in which a diploid cell is reduced to two haploid daughter cells that contain only one of each pair of homologous chromosomes. During meiosis
II, those haploid cells are further divided to form four haploid cells. Spermatogenesis in males produces four viable sperm cells from each complete cycle of meiosis. Oogenesis produces four daughter cells, but only one is a viable egg and the other three are polar bodies.

Fertilization
Fertilization is the union of a sperm cell and an egg cell to produce a zygote. Many sperm may bind to an egg, but only one joins with the egg and injects its nuclei into the egg. Fertilization can be external or internal. External fertilization takes place outside of the female's body. For example, many fish, amphibians, crustaceans, mollusks, and corals reproduce externally by spawning or releasing gametes into the water simultaneously or right after each other. Reptiles and birds reproduce by internal fertilization. All mammals except monotremes (e.g. platypus) reproduce by internal fertilization.

Embryonic Development
Embryonic development in animals is typically divided into four stages: cleavage, patterning, differentiation, and growth. Cleavage occurs immediately after fertilization when the large single-celled zygote immediately begins to divide into smaller and smaller cells without an increase in mass. A hollow ball of cells forms a blastula. Next, during patterning, gastrulation occurs. During gastrulation, the cells are organized into three primary germ layers: ectoderm, mesoderm, and endoderm. Then, the cells in these layers differentiate into special tissues and organs. For example, the nervous system develops from the ectoderm. The muscular system develops from the mesoderm. Much of the digestive system develops from the endoderm. The final stage of embryonic development is growth and further tissue specialization. The embryo continues to grow until ready for hatching or birth.

Postnatal Growth
Postnatal growth occurs from hatching or birth until death.
The length of the postnatal growth depends on the species. Elephants can live 70 years, but mice only about 4 years. Right after animals are hatched or born, they go through a period of rapid growth and development. In vertebrates, bones lengthen, muscles grow in bulk, and fat is deposited. At maturity, bones stop growing in length, but bones can grow in width and repair themselves throughout the animal's lifetime, and muscle deposition slows down. Fat cells continue to increase and decrease in size throughout the animal's life. Growth is controlled by genetics but is also influenced by nutrition and disease. Most animals are sexually mature in less than two years and can produce offspring.