Answer: Fertilization >> Cleavage >> Gastrulation >> Organogenesis
Explanation:
While differences are seen depending on the species under study, embryonic development follows a relatively stereotyped pattern, as follows:
1) Fertilization: The zygote (first cell, technically an unicellular embryo) is formed by the fusion of the male and female gametes.
2) Cleavage: The zygote undergoes a series of cell divisions, resulting in a multicellular embryo (usually called a blastula).
3) Gastrulation: The blastula undergoes a series of transformations (mediated by processes like cell differentiation, migration, and/or shape change) which results in the acquisition of core anatomical features such as the main germ layers (ectoderm and endoderm, mesoderm if present) and rudiments of the digestive tract.
4) Organogenesis: After gastrulation, the embryo has acquired an anatomical outline reminiscent of the mature individual. Now, specific organs begin development from precursor populations called “primordia”.
Note that, in practice, the aforementioned stages may temporally overlap.
