Human embryonic development or human embryogenesis refers to the development and formation of the human embryo. In sexually reproducing organisms, embryonic development begins just after fertilization and continues through the formation of body structures, such as tissues and organs.Complete Answer:
Each embryo starts its development as a zygote which is a single cell resulting from the fusion of gametes. In the first stages of embryonic development, a single-celled zygote undergoes several rapid cell divisions, called cleavage and forms a blastula. Next, the cells in a blastula-stage embryo start rearranging themselves into layers. That process is called gastrulation. These layers each give rise to different parts of the developing multicellular organism, such as the nervous system, connective tissue and organs. Early human embryo possesses a dorsal hollow nerve cord, a well-developed notochord and a series of gill slits to sustain itself in the environment of the womb. These slits are found in embryos of all vertebrates because they share the common ancestry of fish in which these structures first evolved. In humans, they disappear before birth.Note:
Between the head and the heart, a series of branchial arches, i.e., cartilaginous structures supporting the gills of fishes and larval amphibians, begin to form. In higher vertebrates, these structures form part of the jaw and the ear.
Figure: Gill slits in an embryo