1. Fertilization: The process where a sperm cell unites with an egg cell, forming a single cell called a zygote. This marks the beginning of a new individual.
2. Cleavage: The zygote undergoes rapid cell division (mitosis) without significant growth. This forms a ball of cells called a morula, which then hollows out to form a blastula.
3. Gastrulation: The blastula undergoes a significant reorganization where cells migrate and form three germ layers:
* Ectoderm: Develops into the outer layer of the body, including skin, nervous system, and sense organs.
* Mesoderm: Forms the middle layer, including muscle, bone, blood, and connective tissue.
* Endoderm: Forms the inner layer, including the lining of the digestive system, lungs, and other internal organs.
4. Organogenesis: The germ layers differentiate into specific tissues and organs. This is where the basic body plan of the animal is established.
5. Growth and Differentiation: The organism continues to grow and mature, with cells specializing into various functions. This stage involves significant increases in size and complexity.
6. Metamorphosis (in some animals): A dramatic transformation in body form, often involving shedding of the skin or exoskeleton. This is characteristic of insects, amphibians, and some other species.
7. Sexual Maturity: The organism reaches reproductive capacity and can participate in the cycle of life.
Specific Examples:
* Mammals: Development occurs within the mother's womb (gestation), and young are often born in a relatively advanced state.
* Birds: Development occurs within an egg, with the embryo receiving nourishment from the yolk. Hatchlings are typically less developed than mammalian newborns.
* Amphibians: Eggs are laid in water, and development involves a larval stage (tadpole) that undergoes metamorphosis into an adult frog or toad.
* Insects: Development often involves distinct larval stages (e.g., caterpillars) followed by metamorphosis into a pupal stage and ultimately, the adult form.
It's important to remember that this is a very simplified model. Development in animals is incredibly diverse and can be influenced by many factors, including genetics, environment, and parental care.