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All these essential parts of the middle ear originate from the first gill-cleft and its surrounding part; in the Selachii this remains throughout life an open squirting-hole, and lies between the first and second gill-arch. In the embryo of the higher Vertebrates it closes up in the centre, and thus forms the tympanic membrane.
Above at the basis a small process grows out of this first gill-arch; this is the upper-jaw process. The upper-jaw process forms the chief part of the skeleton of that jaw, the palate bone, and the pterygoid bone.
The following characteristics of the Gnathostomes are anatomic features of this kind: The internal gill-arch apparatus with the jaw arches; the pair of nostrils; the floating bladder or lungs; and the two pairs of limbs. It is inherited in rudimentary form by all of them, from the earliest fishes to man.
The outlying remainder of the first gill-cleft is the rudiment of the external meatus. From its inner part we get the tympanic cavity, and, further inward still, the Eustachian tube. Connected with this is the development of the three bones of the mammal ear from the first two gill-arches; the hammer and anvil are formed from the first, the stirrup from the upper end of the second, gill-arch.
Head of a chick embryo, four days old, from below. n nasal pit, o upper-jaw process of the first gill-arch, u lower-jaw process of same, k double apostrophe second gill-arch, sp choroid fissure of eye, s gullet. Heads of chick embryos: 2.309 from the end of the fourth, 2.310 from the beginning of the fifth week. There is little to be said of the development of the lower sense-organs.
From the first gill-arch, from the inner surface of which the muscular tongue proceeds, we get the first structure of the maxillary skeleton the upper and lower jaws, which surround the mouth and support the teeth. These important parts are wholly wanting in the two lowest classes of Vertebrates, the Acrania and Cyclostoma.
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