Embryogenesis is the process by which a fertilized egg, or zygote, develops into a multicellular embryo. It encompasses a series of intricate and highly coordinated cellular and developmental events that lead to the formation of the basic body plan and structures of an organism. Embryogenesis is a crucial phase in the life cycle of all sexually reproducing organisms and sets the stage for the later development of the individual. The specific details of embryogenesis can vary significantly between species, but there are some common key stages and processes involved:

1. Fertilization: Embryogenesis begins with fertilization, where a sperm cell from a male fuses with an egg cell (ova or ovum) from a female to form a diploid zygote. This zygote contains the full complement of genetic information (chromosomes) needed to develop into an entire organism.
2. Cleavage: After fertilization, the zygote undergoes a series of rapid cell divisions known as cleavage. During cleavage, the size of the individual cells decreases, and the embryo becomes a multicellular structure called a morula.
3. Blastula Formation: The morula continues to divide and transform into a hollow sphere of cells called a blastula. The hollow cavity within the blastula is called the blastocoel.
4. Gastrulation: Gastrulation is a fundamental process where the blastula reorganizes into a three-layered structure called the gastrula. The three primary germ layers are the ectoderm, endoderm, and mesoderm. These layers give rise to different tissues and organs in the developing embryo.
5. Neurulation (in vertebrates): Neurulation is the process in which the neural tube is formed from the ectoderm. The neural tube eventually becomes the central nervous system, including the brain and spinal cord.
6. Organogenesis: During this phase, the organs and organ systems of the embryo develop from the germ layers. Specific tissues and structures differentiate and take on their roles in the organism.
7. Fetal Development: Embryogenesis transitions into fetal development as the embryo continues to grow and differentiate into a more complex organism. This phase includes the development of specialized tissues, organs, and systems, as well as the maturation of the organism.

Embryogenesis can vary in duration, complexity, and specific details depending on the species. For example, the embryogenesis of a human is more complex and extended than that of a simpler organism like a fruit fly. The process is tightly regulated by a combination of genetic factors and environmental cues to ensure the proper development of an organism. Errors or disruptions during embryogenesis can lead to developmental abnormalities or birth defects.