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Amniotes do not have gills. The gill arches form as pharyngeal arches during embryogenesis, and lay the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals, the malleus and incus. [2]
Some fish species have a second set of jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation. [6] During embryonic development, a series of pharyngeal arch pairs form. These project forward from the back of the embryo toward the front of the face and neck.
Human embryonic development covers the first eight weeks of development, which have 23 stages, called Carnegie stages. At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs.
Pharyngeal clefts resembling gill slits are transiently present during the embryonic stages of tetrapod development. The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that " ontogeny recapitulates phylogeny "; this hypothesis, while false, contains elements of truth ...
The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the developing mammalian embryo at this stage is called a blastocyst, not a blastula, which is more generic term. There are also several other differences from embryogenesis in lower chordates.
At the pharyngula stage, all vertebrate embryos show remarkable similarities, i.e., it is a "phylotypic stage" of the sub-phylum, [3] containing the following features: notochord; dorsal hollow nerve cord; post-anal tail, and; a series of paired branchial grooves. The branchial grooves are matched on the inside by a series of paired gill pouches.
The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present. [8] * Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
An embryo is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle ...