[QUOTE=Frelling Tralk]
I don’t see how Blake’s point has anything to do with my question, which is:
Is there actually a structural difference between what are called “gill slits” in embryonic fish and “Pharyngeal arches, clefts, and pouches” in human embryos, or is it just that they use different terminology to describe them because of what they eventually become?
[/QUOTE]
Some of each. To be fair, *pharyngeal *arches/clefts/pouches are also often referred to as *branchial *arches/clefts/pouches. Branchial comes from the Greek for gills. Strictly speaking, I suppose one would reserve the term branchial for those structures that would give rise to gills, but there is overlap in the terminology, so one finds references in the human medical literature to branchial cleft cysts, fistulas, etc. So the terminology is at least theoretically different based on what the tissues will eventually become.
But there are structural differences as well. In both fish and humans, as the embryo develops, a characteristic set of alternating ridges and grooves forms on the outside of the embryo, while simultaneously inside the embryo’s gut tube a series of pouches form - wide spots in the gut tube. The ridges are the pharyngeal/branchial arches, the grooves are the clefts, and the pouches are the, uh, pouches. In between the embryo’s outer covering of ectoderm and the endoderm which lines the gut is mesoderm. In humans, the grooves deepen and the pouches enlarge and the endoderm thins out to the point that the ectoderm is in contact with the endoderm in places. The mesoderm soon moves back in and separates the two layers, however. In the case of the human ear, the first cleft deepens and contacts the first pouch. The tissue of the cleft forms the outer ear canal and eardrum. The mesoderm of the first and second arches that moves back into place between the two layers forms the bones of the middle ear, and the first pouch forms the lining of the middle ear and the Eustachian tube.
In the case of fish, the grooves deepen and the pouches expand, and the endoderm and ectoderm come in contact and then break down, so there is open communication between the pouch and cleft. This is a gill slit. The branchial tissues then go on to form the jaw and gills of a fish.
That’s a long way to go just to arrive at the statement that humans don’t have gill slits. Humans don’t have gills at any point in embryonic development. They do, however, share a basic vertebrate body plan with fish that says that ‘things between the head and heart will form from this wrinkly patch here, that from the outside bears a superficial resemblance to the gills of a mature fish’. Humans and fish also form eyes and brains from the same basic precursor structures, but we don’t form fish eyes or fish brains, either.