Early embryological studies
During the four decades after Cuvier, embryological studies of all classes of vertebrates clarified the homologies of the middle ear structures, which, prior to Owen (1843), were more usually referred to as analogies. Carus (1818) had doubted that the articulation of the lower jaw is identical between mammals and the other vertebrates, and he recognized that the incus (anvil) is homologous to the quadrate of ‘lower’ (i.e. non‐mammalian) vertebrates. Meckel (1820) observed that, in mammals, the embryonic malleus (hammer) develops from the posterior end of a thin rod of cartilage attached to the medial side of the dentary, whereas in non‐mammalian vertebrates this posterior end ossifies as the articular bone; Meckel therefore homologized the malleus with the articular. The embryonic cartilage of the lower jaw is now called ‘Meckel's cartilage’ after its discoverer. These discoveries replaced some earlier homologizations, such as those of Geoffroy‐Saint‐Hilaire (1818).
These finds gained a new importance when Rathke (1825a,b) described Kiemenspalten (gill slits) and Kiemenbögen (gill arches) in embryos of the pig and chick. The discovery of Kiemen‐Anlagen (gill anlagen) stimulated a number of similar studies on many different amniotes, including man. These early studies suggested that the most anterior of the Kiemenbögen represents the mandibular arch and that the second represents the hyal arch (Baer, 1828). The term Kiemenbogen was accordingly replaced by the more general terms Visceralbogen (visceral arch) or Schlundbogen (pharyngeal arch). Huschke (1827, 1828) concluded that the first cleft between the mandibular and the hyal arches (Spritzloch = spiraculum) corresponds to the ear duct (Gehörgang = Eustachian tube, middle ear, and outer ear).
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The Reichert–Gaupp theory
Carl Reichert, who had mainly studied the embryogenesis of the second visceral arch (cf. cartilage of Reichert), in 1837 summarized and generalized his own findings and those of colleagues such as Meckel (1820), Carus (1818), Rathke (1825a,b) and Baer (1828). Since then these concepts have (not quite correctly) been collectively referred to as ‘Reichert's theory’. The numerous and often controversial publications were carefully reviewed and summarized by Gaupp (1898), Broman (1899), and Van der Klaauw (1924).
The homology of the incus and malleus with the quadrate and articular, respectively (Fig. 1A) was accepted by most authors, but Huxley (1869) and Parker (1874, 1877) maintained different views: they derived only the malleus from the mandibular arch and aligned the incus with the second visceral or hyal arch. Establishing the homology of the stapes proved to be more complicated, the more so because it was difficult to compare it among different tetrapod groups. Reichert (1837) had homologized the stapes with the columella of amphibians, but he did not precisely define what he meant by ‘columella’ because he did not distinguish an extracolumella (Reichert, 1838). Huxley (1869) and Parker (1874, 1877) considered the stapes as an element isolated from the lateral wall of the labyrinth capsule (see section below on developmental biology). Broman (1899) showed conclusively that the human stapes chondrifies as an ‘annulus stapedius’, which is in contact with the hyal arch (cartilage of Reichert) by means of an interhyal blastema. The ‘annulus stapedius’, which often forms around the stapedial artery, comes into contact with the wall of the labyrinth only later and forms a fenestra ovalis (vestibuli) secondarily. Therefore, embryological evidence clearly supported the case that the stapes is the most proximal portion of the hyal arch (Fig. 1A, see also section on developmental biology). Gaupp (1898) adopted this view contra his earlier viewpoints and these homologies of the three middle ear ossicles were also accepted by Gegenbaur (1898).
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