Human Development Timeline: Zygote is formed at 30 hours, Morula is formed between 30 hours and Day 4, Blastula is formed from Day 4 to 7, Gastrula is formed from Day 7 to Week 3, Embryo is formed from Weeks 3 to 8, and Foetus is formed from Weeks 8 to Birth.
Fertilisation occurs 12 - 24 hours after ovulation in the fallopian tube, with the penetration of the oocyte using acrosomal enzymes in the head, sperm dissolves and penetrates the zona pellucida (secondary oocyte’s).
The initial event in embryonic development is Primitive streak formation, a morphological groove formed by migrating epiblast cells, which will form two germ cell layers (endoderm and mesoderm) and the embryo body axis (head-tail/left-right orientation).
Thickening of endometrial layers is due to extra-embryonic mesoderm, which fills the space between trophoblast, amnion, and chorion, forming an extra-embryonic coelom.
The embryonic disc now has endoderm, mesoderm and ectoderm, and this flat disc begins to fold ventrally at the edges, to form a hollow cylinder (tube) of the three germ layers.
The two parts derived from trophoblast are the extraembryonic mesoderm and coelom, which surrounds the embryo and later the foetus, becomes part of the placenta.
Gastrulation is the major event in mammalian embryonic development where the bilaminar embryonic disc is reorganised into a trilaminar structure, containing three germ layers, which will form the embryo's entire body organs and tissues.