Zygote: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.