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ORGANOGENESIS 419 sus, hyoglossus, and intermandibularis, the dentary, splenial, and angular bones, Meckel's cartilages, lingual lipid, and fibrous tissue appear and are patterned comparable to those seen in vivo (Figs. 29A and B; Ferguson et al., 1982, 1983). However, anlagen for taste buds and teeth are absent. The differentiation of alligator branchial arches in vitro is superior to that for any other vertebrate studied to date, so that they are useful models for investigation of a variety of developmental phenomena. B. Fses snd Noss There are accounts of the development of the face and nose for several genera: Alligator (Rathke, 1866; Clarke, 1891; Reese, 1901b, 1908, 1915a, 1925; Bertau, 1935; Wettstein, 1954; Parsons, 1959a, 1970; Ferguson, 1981a, b, 1982b, 1984a), Caiman (Rathke, 1866; Parsons, 1970; Saint Girons, 1976), Melanosuchus (Bertau, 1935; Parsons, 1970) and Crocodylus (Rathke, 1866; Meek, 1893, 1911; Rose, 1893a, b; Seydel, 1896, 1899; Voeltzkow, 1899; Bertau, 1935; Wettstein, 1954; Wegner, 1957; Parsons, 1959a, b, 1970; Guibe, 1970m; Saint Girons, 1976; Bellairs, 1977; Ferguson, 1984a). That of Cavialis is unknown except for a figure in Bruhl (1866), an illustration by Bustard (1980c) and a description of the nasal excrescence (Martin and A. d'A. Bellairs, 1977). Butler (1905) notes that the snout of either Cavialis or Tomistoma is shorter and wider in the embryo than in the adult (but see (, Fig. 27. Alligator mississippiensis. Embryos. (A-D) and (F-G); Crocodylus lliloticus. (E-I). (A) Stage 7. Scanning electron micrograph (SEM) of the cranial aspect. Note the first and second branchial arches and first branchial cleft (arrowed). (B) Stage 8. Face-on view. Note the midline fissure and facial processes. (C) Stage 9. Scanning electron micrograph. Note the four branchial arches, grooves and clefts, nasal pit, limb buds, and tail. (D) Higher power view of C illustrating the true 1st and 3rd branchial arch clefts (arrowed), the four branchial arches and the branchial sinus. The apparent cleft between the 2nd and 3rd arches is an artifact. (E) Stage 10. Note the nasal pits and surrounding nasal processes, the lens and surrounding optic cup, the second branchial arch starting to overgrow arch 3, the 1st branchial cleft ventral to the otocyst and the brain regions. (After Voeltzkow, 1899.) (F) Stage 14. SEM illustrating closure of the nasal pit slits by the medial nasal, lateral nasal and maxillary processes. Note the intraoral bulges of the club shaped maxillary processes, signifying the onset of secondary palate development, the anterodorsal elevations of the medial nasal processes and the enlarging mandible. Compare with Figure D. (G) Stage 9. Horizontal hematoxylin and eosin section through the five branchial arches (1-5). Note the thick endoderm, thinner ectoderm, branchial grooves and pouches, aortic arch arteries,and branchiomeric nerves. (H, I) Stages 12 and 15. Two diagrams illustrating the rearrangement of the first branchial cleft to form the external ear and superior ear flap. (After Voeltzkow, 1899.) A, Auricular hillocks; AA, aortic arch arteries; AD, anterodorsal elevations of nasal processes; BN, branchiomeric nerves; BP, branchial pouches; BS, branchial sinus; e, first branchial cleft; 0, denticles; E, eye; Ee, ectoderm; EN, endoderm; EX, external surface. F, ear flap; FL, fore limbbud; G, branchial grooves; H, hindbrain; HL, hind limbbud; LN, lateral nasal process; M, midbrain; MA, mandibular process; MD, mandible; MN, medial nasal process; MX, maxillary process; N, nasal placode; NP, nasal pit; 0, optic placode; Oe, optic cup; OT, otocyst; P, palatal shelves; PE, pericardial sac; PH, pharynx; 1, 2, 3, 4, branchial arches 1-4.
420 REPROOUCTIVE SIOLOGY ANO EMSRYOLOGY OF CROCOOILIANS Bustard, 1980c for a photograph of a Cavialis hatchling with a long thin snout). The literature on crocodilian nasal embryology has been reviewed by Parsons (1959a, 1970). The buccopharyngeal membrane initially separates the surface ectoderm from the pharyngeal endoderm (Parker, 1883). After its rupture, the primitive mouth is bounded by the forebrain above and the pericardial sac below. The differentiation of Rathke's pouch and the pituitary is discussed in Sections VII.I and P and for reptiles in general by Pearson, Chapter 9, this volume. Facial development in Alligator mississippiensis (Ferguson, 1981a, b, 1982b, 1984a), Crocodylus johnsoni and C. porosus (Ferguson, 1984a) may be summarized as follows (for chronology, see Section VI). The first sign of nasal development is the appearance of bilateral epithelial thickenings on the anterolateral aspects of the forebrain bulge (Fig. 27A). These nasal placodes invaginate toward the roof of the primitive oronasal cavity and form the nasal pits (Fig. 27A). As the nasal pits invaginate, two rims of tissue are embossed outward around their surface openings to form the medial nasal and lateral nasal processes (Figs. 24B, 27B and F). The area between the two nasal pits (including the medial nasal processes of each side) is known as the frontonasal process and is divided by a medial groove (Fig. 24B). Bilateral maxillary processes arise from the dorsal ends of the mandibular arch (Figs. 24A-C, 27A, B, and F) and grow forward from the angles of the primitive mouth cavity to form progressively more of its cranial borders (Figs. 19, 24A-D, 27A, B, and F). The club-shaped maxillary processes form the lower boundaries of the developing eye (Figs. 19, 24A-D, and 27F). Mesenchyme in the maxillary processes derives principally from the mesencephalic neural crest, whereas that in the medial and lateral nasal processes derives principally from the prosencephalic neural crest. The maxillary mesenchymal cells migrate along several routes. Extra-orally they migrate forward to form an ever increasing portion of the upper margin of the stomodeum; also, they pass between the developing eye and ear and between the developing eye and lateral nasal process, demarcating the naso-optic furrow between the lateral nasal and maxillary processes. The furrow eventually closes entrapping an epithelial rod, which canalizes to form the naso-Iacrimal duct. Intra-orally the maxillary processes have two principal advancing fronts of migrating mesenchymal cells, namely the tectoseptal processes and the palatal shelves (Section VILC). The invaginating nasal pits fuse with epithelium in the roof of the oronasal cavity so forming the primitive nasal cavities with anterior and posterior choanae. The posterior choanae open into the roof of the mouth a few millimeters behind the anterior choanae. By a combination of extremely rapid outward proliferation of the medial nasal and lateral nasal processes and a caudal extension of nasal pit invagination, the nasal pit slits develop between the ipsilateral medial and lateral nasal processes ORGANOGENESIS 421 (Figs. 24A and B). The club-shaped maxillary processes come to lie beneath the nasal pit slits, which are eventually closed as the maxillary processes unite with the lateral and medial nasal processes (Figs. 240 and 27F). This uniting involves limited cell death and extensive migration of epithelial cells, which exhibit a characteristic cobblestoned, villous morphology. Throughout this period, differential growth of the head brings the nasal pits and related processes from their early lateral positions (Figs. 27A and B) closer to one another in the midline (Figs. 24A-D and 27F). The adjacent medial nasal processes develop paired intraoral projections, which merge to form the primary palate and primary nasal septum. Differential growth of extraoral caudal elevations of the medial nasal processes shifts the anterior nasal choanae from the tip of the snout to the adult dorsal position (Figs. 240 and E, 27F). The posterior nasal choanae now open behind the primary palate. The development of the secondary palate and associated structures is discussed in Section VII. C. Later, cartilage, bone, and muscle differentiate in various parts of the facial and nasal skeleton, the nasal epithelium becomes regionally specialized, the anlagen of the nasal glands and ducts appear, and the complex conchae and recesses, which characterize the crocodilian nose, begin to form (Bertau, 1935; Parsons, 1959a, 1970; Saint Girons, 1976). c. Palate snd Nasopharyngeal Duct Unlike any other reptiles, adult crocodilians have a mammal-like secondary palate composed of the sutured palatal processes of the maxillary, palatine and pterygoid bones (His, 1892; Moll, 1888; Busch, 1898; Gappert, 1903; Voeltzkow, 1903b; Hoffman, 1905; Fuchs, 1907, 1908, 1910; Sippel, 1907, Fleischmann, 1910; Thater, 1910; Lakjer, 1927; Barge, 1937; Peter, 1949; A. d'A. Bellairs and Boyd, 1957; Pasteels, 1950; Guibe, 1970b; Iordansky, 1973; Ferguson, 1981a, b, 1982b, 1984a). It is even unlike the avian palate, which is muscular and permanently cleft (Sippel, 1907; Barge, 1937; Pasteels, 1950; A. d'A. Bellairs and Jenkin, 1960). Crocodilian embryos are therefore useful in studies of normal and abnormal craniofacial development, largely because of their accessibility to experimental manipulations inside the egg (Ferguson 1979a, 1981a, b, 1982b, 1984; Ferguson and Honig, 1984; Ferguson et al., 1981). Some macroscopic drawings of the palates of embryos appear in His (1892), Voeltzkow (1899), Gappert (1903), Sippel (1907), and Fleischmann (1910), but no histological details of palatogenesis are given. More detailed studies of the development of the nasopharyngeal duct by Fuchs (1908) and Muller (1965, 1967) contain errors (see below; Ferguson, 1981b, 1984a). The following account is based on investigations of Alligator mississippiensis (Ferguson 1981a,b,d, 1982b, 1984a; Ferguson et al., 1984; Ferguson and Honig, 1984) and of Crocodylus johnsoni and C. porosus (Ferguson, 1984a). Description of the early development of the alligator face (Section VILB)
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