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Fig. 25. Alligator mississippiensis. (A) Stage 17. View of the inside of the mouth (lower jaw and tongue removed). Note the closed primary palate, the secondary palatal shelves, which have grown out from the maxillary processes on each side but which do not yet contact each other, the closing tectoseptal processes, caruncle and denticles. (B) Early Stage 18. Palatal view. The secondary palate is half closed. Note the V-shaped margins of the approximating palatal shelves. (C) Late Stage 18. Palatal view. The palate is three-quarters closed. (0) Stage 19. Palatal view. The secondary palate and tectoseptal processes are almost completely closed. Note also the distinct anterior nictitating membrane of the eye and lower eyelid. (E) Early Stage 20. Palatal view. The palate is completely closed but the basihyal valve has not yet developed. (F) Stage 21. Palatal view. Note the development of the palatal plexus of blood vessels and the basihyal valve. (G) Stage 23. Palatal view. Note the extensive palatal plexus of blood vessels and the outlines of the sensory papillae along the jaw margins. (H) Stage 19. View of the tongue and lower jaw. Note the denticles and teeth, epiglottis and swellings of Fig. 26. Alligator mississippiensis. Views of the snout to show the development of the caruncle. (A) Late Stage 18. Frontal view. Caruncle is represented by two widely spaced elevations. The lower jaw has been removed. (B) Stage 19. Ventral view. Note that the two white blobs of the caruncle have joined in the midline. (C) Stage 20. The caruncle now appears more solid. (0) Stage 21. (E) Stage 23. Note the smooth base of the bifid caruncle and elevations for the pigmented sensory papillae (S) on the jaw margins. (F) Stage 25. Note that the smooth base of the caruncle is pigmented. (G, H) Ventral and frontal views of Stage 27. E, Caruncle; S, sensory papillae. the paired hypobranchial eminences which will form the lower flap of the basihyal valve. (I) View of the tongue and lower jaw of Stage 22. Note the plexus of blood vessels in the lower jaw and the lower flap of the basihyal valve. A, Anterior nictitating membrane; B, basihyal valve; E, epiglottis; Ee, caruncle; D, denticles; H, hypobranchial eminences; L, lower eyelid; PP, primary palate; P, secondary palate; T, tectoseptal processes.
416 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS ORGANOGENESIS 417 wall to enclose the embryonic intestines, yolk sac and yolk; it ends with the absorption of the latter into the abdominal cavity. Ventral skin forms across the yolk scar. Stage 28 [Figs. 21 and 23] At Stage 28 the yolk scar diminishes in length and width as the absorbed abdominal yolk is utilized. There are considerable variations both among and within species as to the volume of absorbed yolk and the size of the yolk scar at hatching (which occurs at the end of this stage). These parameters may be influenced by the temperature of egg incubation (Ferguson and Joanen, 1982, 1983). The growth and changing proportions of embryos of A. mississippiensis from 12 days after egg laying to hatching are illustrated in Fig. 22. VII. ORGANOGENESIS A. Branchial Arches In Alligator mississippiensis (Parker, 1883; Clarke, 1891; Reese, 1908, 1912, 1915a; Ferguson, 1984a), Crocodylus johnsoni (Ferguson, 1984a), C. porosus (Ferguson, 1984a), and C. niloticus (Voeltzkow, 1899), the five branchial arches appear in a cranio-caudal sequence (described in Section VI), beginning as a series of thickened epithelial involutions in the pharynx (Figs. 27C, E, and G). Initially, these branchial pouches are internal grooves lined by thickened epithelium but they soon approximate the external branchial grooves. The pharyngeal endoderm fuses with the surface ectoderm to form true clefts in the dorsal half of the junctions between the 1st and 2nd and the 3rd and 4th arches (Ferguson, 1984a; Figs. 18, 19, 27C, 0, and G). In the remaining junctions, thin membranes of ectoderm, mesoderm, and endoderm separate the external grooves from the internal pouches (Ferguson, 1984a; Figs. 18, 19, 27C, 0, and G). The first branchial cleft migrates dorsally until it overlies the otocyst near which it forms the external meatus of the ear (Ferguson, 1984a, Figs. 27H and I). Auricular hillocks develop along its ventral margin, which becomes the superior flap of the external ear (Figs. 27H and I). The third branchial cleft is transitory. Its epithelia become specialized and develop numerous cilia and cell processes, which extend from epithelial cells on one side of the cleft to those on the other (Ferguson, 1982b, 1984a). In this way, the two epithelia oppose one another, fuse, and migrate, thus closing the branchial clefts (Ferguson, 1982b, 1984a). In fish, patent branchial clefts separate all the arches to form gills, whereas mammals lack patent branchial clefts (Sperber, 1981). The limiting membrane becomes the tympanum in the remodeling of the first branchial groove and pouch in mammals; tympanic development in crocodilians has not been reported. The significance of the transitory opening of the third cleft is unknown. The branchial sinus forms at the base of the most caudal branchial arch (Fig. 270); its specialized epithelium invades the underlying pharyngeal mesoderm (Ferguson, 1982b, 1984a). Each branchial arch is covered externally by ectoderm and internally by endoderm, which extends to the outer border of the mandibular arch (Fig. 27G) and consists of a central core of mesoderm surrounded by a mass of mesenchymal cells derived from the neural crest (Ferguson, 1981a). Preliminary experiments, involving surgical excision or chemical destruction of alligator neural crest cells and heterotypic grafting of quail neural crest cells at different levels of the neuraxis (Ferguson, 1981a, 1982b, 1984a, unpubl.) suggest that neural crest cells migrate in temporally distinct waves from different levels of the neuraxis. Crest cells destined for the mandibular arch and its maxillary processes migrate from the rostral and caudal levels of the mesencephalon, respectively (Ferguson, 1981a). It is therefore possible selectively to block the migration of various waves of neural crest cells to produce alligators with normal maxillary processes and therefore a normal upper jaw and palate, but with virtually no lower jaw or tongue (Fig. 37E; Ferguson, 1981a, 1984a). This technique, combined with shell-less, or semi-shell-Iess culture, has enabled palatal development to be viewed as it happens; this is the first such longitudinal study in any animal (Ferguson, 1981a, 1982b, 1984a). Each branchial arch also contains its own aortic arch artery and appropriate cranial nerve (Figs. 18, 19, and 27G). The neural crest cells appear to give rise to all of the skeletal and connective tissues of the face, apart from striated muscle which is derived from the arch mesoderm; these preliminary data (Ferguson, 1981a, 1982b) indicate that the general pattern of neural crest derivatives in alligators is comparable to that in birds (LeOouarin, 1982). Macroscopic descriptions of branchial arch development and closure of the cervical sinus are given in Section VI and in Ferguson (1984a). Descriptive accounts of the branchial pouch derivatives and the cartilaginous, bony and muscular derivatives of the branchial arches exist for several species and include comparative comments with other vertebrates (Parker, 1883; Voeltzkow, 1903a,b; Reese, 1908, 1915a; Edgeworth, 1907; Goodrich, 1930; Pasteels, 1950; Oalcq and Pasteels, 1954). First branchial arches excised from Stage 10 alligator embryos can be cultured in vitro at 30°C and 37°C (Ferguson et al., 1981, 1983). In general, chemically defined, serumless media at 37°C are useful for short-term (up to 14 days) studies, whereas serum supplemented media at 30°C are useful for long-term (up to 60 days) studies. Explanted arches consist initially of lobulated cylinders of undifferentiated mesenchyme covered by epithelium (Fig. 27G). They undergo normal morphogenesis and differentiation in culture (Figs. 29A and B; Ferguson et al., 1982, 1983). Paired lingual swellings form a tongue-like structure. Blastematae for the Mm. genioglos
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