Sexual Reproduction

Male gamete + Female gamete → Zygote n + n → 2n Fertilization Fertilization is the union (fusion) of a monoploid sperm cell and monoploid egg cell. In fertilization, the resulting diploid zygote has a species number of homologous chromosomes (2n) restored. Male gamete + Female gamete → Zygote n + n → 2n Fertilization must occur in a moist environment.

Types of Fertilization External Fertilization: Fertilization takes place outside the body in water. Usually found in aquatic animals like fish and amphibians. Usually large numbers of eggs. Large numbers of offspring due to low survival rates. Internal Fertilization: Fertilization takes place in the moist reproductive tract of the female. Found in most terrestrial vertebrates. Birds, reptiles, and mammals. Usually smaller numbers of offspring. Often times more parental involvement.

Embryonic Development Cleavage: series of rapid mitotic cell divisions following fertilization. Increase in numbers of cells with very little growth in size. Once cleavage begins the zygote becomes a developing embryo. Followed by mass of cells called the morula. Followed by hollow ball of cells called the blastula.

Gastrulation Gastrulation: The blastula becomes indented forming the gastrula. 3 Layers: Endoderm (inner layer) Mesoderm ( middle layer) Ectoderm (outer layer)

Diagram of Early Development

Differentiation -Differentiation: The three embryonic layers differentiate and give rises to various tissues, organs, and systems of multicellular animals. Ectoderm: Skin and nervous system Mesoderm: Muscles, circulatory system, skeleton, excretory system, and gonads. Endoderm: Lining of digestive and respiratory tracts, and portions of the liver and pancreas. -Growth: Embryo increases in cell number and size.

Development Site of Development External Development: Occurs outside of female’s body. In Water (aquatic environment): The eggs of many fish and amphibians are fertilized externally and develop externally in an aquatic environment. Survival rates are usually low Embryo’s source of food is the yolk stored in the egg.

External Development (cont’d) On Land (terrestrial environment): Eggs of birds, many reptiles, and a few mammals develop on a land environment after internal fertilization. Usually fewer number of fertilized eggs because of better survival rate. Developing embryo’s food source is yolk.

Adaptations For Land Development Shell provides protection to the egg and helps keep moisture in. Membranes: Parts of the egg with different functions: Amnion: Contains amniotic fluid which protects embryo from shock. Yolk sac: Surrounds yolk which provides food. Allantois: Respiratory membrane and stores uric acid which is a nitrogenous waste. Chorion: Outer membrane surrounding the other membranes and separates them from the environment.

Fertilized Bird Egg A- Yolk Sac B- Egg Shell C- Chorion D- Embryo E- Amnion F- Allantois

Internal Development Internal Development involves the growth of the embryo within the body of the parent. Parent usually provides the nutrition. Relatively high survival rates, few fertilized eggs, most often parental care.

Placental Mammals Placental Mammals: Examples are humans, cows, etc. Have internal fertilization. Embryo(s) develop internally in an organ called the uterus. Eggs of mammals have relatively little yolk. The placenta, a specialized organ in the uterus is formed from embryonic and maternal tissues. Nutrients, wastes, and respiratory gases are exchanged here between the mother and embryo by active transport and diffusion. No direct connection between the two bloodstreams. Connected by umbilical cord.

Placental Development 1- Cervix 2- Umbilical Cord 3- Placental 4- Oviduct 5- Amniotic Fluid

Marsupials Marsupials (non-placental mammals) Internal fertilization and development occur without direct nourishment from the parent. Embryo is born at a relatively premature stage. Completes development in a pouch which contains mammary glands (milk producing) glands.