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INTRODUCTION 331 I. INTROOUCTION II. REPRODUCTIVE BIOLOGY A. General, 333 B. C. CONTENTS Sexual Maturity and Adult Sex Ratios, 333 Courtship, Spermatogenesis, Ovulation, Copulation, the Breeding Season, Fertilization, and Egg Laying, 336 D. Nesting Biology, 349 E. Maternal Behavior, 355 F. Captive Breeding, Egg Collection, and Artificial Incubation, 356 G. Hatching, 360 331 333 P. Endocrine Glands, 445 Q. Thymus and Immune System, 446 R, Limbs and Tail, 446 S, Integument and Its Glands, 450 T. Caruncle, 450 VIII. DEVELOPMENTAL ABNORMALITIES 451 IX. SHELL·LESS, SEMI.SHELL.LESS, AND IN VITRO CULTURE TECHNIGUES 460 X. CONCLUSIONS 462 REFERENCES 464 III. THE EGGSHELL AND SHELL MEMBRANES A. General, 363 363 B. Egg Banding, 363 C. Structure, 367 D. Chemical Composition, 374 E. Water and Gas Conductance and Embryonic Metabolism, 376 I. INTRODUCTION IV. V. VI. VII. THE EGG CONTENTS AND EXTRA.EMBRYONIC MEMBRANES EARLY EMBRYONIC DEVELOPMENT [BEFORE EGG LAYING) STAGES OF EMBRYONIC DEVELOPMENT [AFTER EGG LAYING) ORGANOGENESIS A. Branchial Arches, 41 6 B. Face and Nose, 41 9 C. Palate and Nasopharyngeal Duct, 421 D. Tongue, 427 E. Ear, 428 F. Eye, 428 G. Chondrocranium and Osteocranium, 431 H. Teeth, 431 I. Central Nervous System, 432 ..J. Vertebrae and Ribs, 435 K. Respiratory System, 436 L. Cardiovascular System, 436 M. Diaphragm, 438 N. Gastrointestinal System, 438 O. Urogenital System and Sex Determination, 440 377 381 390 416 I" t I' The living crocodilians, represented by 26 living species placed in three subfamilies (see Table III later in chapter), are the end products of a relatively conservative lineage, which arose from thecodont ancestors approximately 230 million years ago (Carroll, 1969; Walker, 1972). The evolution and taxonomy of the order have been the subject of considerable debate involving paleontological (Steel, 1973), neontological (Wermuth and Mertens, 1977; Groombridge, 1982), immunological, and biochemical approaches (Perutz et al., 1981; Le Clercq et al., 1981; Densmore, 1981; Coulson and Hernandez, 1983). Densmore's and Groombridge's classifications are used here. All types of data indicate that birds are the closest living relatives of crocodilians. However, several aspects of crocodilian embryogenesis, for example, palatogenesis (Section VII, VIII), resemble mammalian phenomena, and variations in hemoglobin amino acid sequences place crocodilians closer to mammals than to snakes (Perutz et al., 1981; Le Clercq et al., 1981; Densmore, 1981). Thus, further studies of crocodilian embryogenesis not only may shed light on fundamental aspects of organogenesis, but also exploit the potential of these vertebrates as models for experimental investigations, which are difficult or impossible to perform in other amniotes (Ferguson, 1981a, b, 1984a). Crocodilian embryology has received very little attention. Some general accounts are available for Crocodylus niloticus (Rathke, 1866; Voeltzkow, 1899, 1901, 1903a), Alligator mississippiensis (Clarke, 1891; Reese, 1908, 1910a-c, 1912, 1915a, b, 1921, 1936), C. palustris, and C. porosus (Derani 330
332 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS REPRODUCTIVE BIOLOGY 333 yagala, 1934, 1936, 1939), whereas Wettstein (1937, 1954) included some embryological data in his general review of crocodilian biology. The present chapter reviews the older, incomplete data and a special effort is made to correct ambiguous or misleading statements or concepts. My studies on shell structure (Ferguson, 1981c, 1982a), sex determination (Ferguson and Joanen, 1982, 1983), and craniofacial development (Ferguson, 1979a, b, 1981a, b, 1982b, 1984a) tend to give the chapter a bias, but an attempt has been made to review all available data in the hope that neglected topics will be pursued by others. Many of the studies on which the present chapter is based have been carried out on the population of Alligator mississippiensis at the Rockefeller Wildlife Refuge in Louisiana, and it might be argued that these animals are not representative of all alligator populations nor of crocodilians in general. However, observations on other populations have revealed no significant differences with respect to basic reproductive biology and embryology. Furthermore, analyses of protein electrophoretic patterns have shown that A. mississippiensis is one of the most genetically homogeneous vertebrate species; protein variations are so low that animals from populations separated by more than 1000 miles cannot be identified as to their geographic source (Gartside et al. 1977; Menzies et al. 1979; Adams et al. 1980). In addition, I have made limited observations on some aspects of the embryology of Crocodylus johnsoni, C. porosus, C. niloticus, C. cataphractus, and C. novaeguineae (Ferguson, 1984a). The basic events during the period of organogenesis (first half of development) are remarkably similar and may be regarded as crocodilian, whereas species-specific differences are more geometrical, for example, variations in the relative sizes and proportions of structures such as the tail, limbs, snout, and pigmentation pattern, and they appear principally during the second half of development. Therefore this account is likely to be representative of crocodilians in general. A recent, dramatic increase in the literature on ecology, behavior, natural history, management, and farming of crocodilians across the world offers hope not only for the survival of these reptiles, but also for the future availability of embryonic study material. Such are particularly exciting prospects not only for the study of neglected species such as the gharial, but also for determining general principles pertaining to crocodilians, and for establishing species-specific variations. Much of this natural history and farming literature is relevant to embrylogical studies. Because it is extremely unlikely that investigations on the "T stages of embryogenesis can be conducted on anything but captive 1
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