The zygote is formed when a sperm cell fertilizes an egg cell, marking the beginning of embryonic development.

Cleavage is the process of rapid cell division that transforms the zygote into a blastocyst, a hollow ball of cells.

The inner cell mass is the group of cells within the blastocyst that will eventually develop into the embryo.

Gastrulation is the process by which the three germ layers are formed, establishing the basic body plan of the embryo.

The ectoderm is the outermost germ layer and gives rise to the nervous system, skin, and other structures.

Neurulation is the process by which the neural tube, the precursor to the central nervous system, is formed from the ectoderm.

The mesoderm is the middle germ layer and gives rise to muscles, bones, connective tissues, and other structures.

Organogenesis is the process by which the various organs and tissues are formed from the germ layers, shaping the overall structure and function of the organism.

Implantation is not a stage of embryonic development but rather the process by which the blastocyst attaches to the uterine wall.

Human embryonic development typically lasts around 40 weeks, from fertilization to birth.

The yolk sac is not an extraembryonic membrane but rather a structure that provides nourishment to the developing embryo.

The placenta facilitates gas exchange, nutrient transport, and waste removal between the mother and the developing embryo.

Human chorionic gonadotropin (hCG) is the primary hormone responsible for maintaining the corpus luteum during early pregnancy.

Implantation is the process by which the embryo attaches to the uterine wall and establishes a connection with the maternal blood supply.

The fetal period is not a critical period during embryonic development, as the major organs and structures have already been formed.