Written by the Fiveable Content Team • Last updated September 2025
Verified for the 2026 exam
Verified for the 2026 exam•Written by the Fiveable Content Team • Last updated September 2025
Definition
An embryo is the early stage of development for a multicellular organism. In humans, it refers to the period from the second to the eighth week following fertilization.
This is the initial cell formed when two gamete cells (sperm and egg) are joined by means of sexual reproduction. It contains all genetic information needed to form a new individual.
Blastocyst: A structure formed in early development, consisting of cells forming an outer shell with an inner cell mass. The blastocyst implants itself into the uterus where it continues developing into an embryo.
Gastrulation: This is a phase early in embryonic development during which basic body plan starts to develop. Three layers (ectoderm, mesoderm, endoderm) that will give rise to different tissues and organs start forming.