Final answer:
Embryonic development starts with fertilization and leads to the formation of the zygote, which undergoes cleavage to form the blastula. Implantation initiates the embryonic stage, where the primary germ layers form and organs start to develop, a process called organogenesis.
Step-by-step explanation:
The early stages of embryonic development begin with the crucial process of fertilization, where a sperm and an egg fuse to form a zygote. Following fertilization, the zygote undergoes cleavage, a series of rapid mitotic cell divisions, which does not increase the total volume of the conceptus but leads to the formation of a blastula.
During the germinal stage, nutrients for this development come from cell cytoplasm or secretions in the Fallopian tube or uterus. The blastula eventually implants in the uterus, marking the transition from the germinal stage to the embryonic stage, and leading to the formation of the primary germ layers: the ectoderm, mesoderm, and endoderm.
Following implantation, gastrulation occurs, where cells differentiate into these germ layers. Soon after, neurulation begins, initiating the central nervous system's development. The embryonic stage is defined by significant growth, changes, and the commencement of organogenesis, where essential organs start to form from the third to the eighth week following fertilization. The heart is one of the first functional organs to develop during this period.
Environmental factors and genetic conditions during the embryonic stage can pose risks to development, potentially leading to birth defects or miscarriage. By the end of this stage, all the organ systems of the embryo are structured in a rudimentary form, setting the foundation for continued development during the fetal period.