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.,........... i calcified layer (0), the honeycomb layer (H) beneath it, and the mammillae. (B) Higher power SEM of the boxed edge seen in (A). Note the granular appearance of the densely packed vertical calcite crystals (the diameter of which is 0.5-1 f.Lm) and the absence of any organic matrix. (C) Erosion crater in the nonopaque zone of an IS-day egg. Note the stepped concentric outline of dissolved crystal layers in the outer densely calcified zone, and the openings of numerous vesicular holes in the exposed porous honeycomb layer at the crater base. (D) Pore in the central opaque zone of a 40-day eggshell. Note the crater-like concentric stepping of the outer densely calcified layers at the pore orifice, remnants of the pore plug and microorganisms (bacterial cocci, rods and filamentous fungae). Outer pore diameter approx. 550 f.Lm, inner pore diameter approx. 100 f.Lm. (E) Fractured edge of a 4-day eggshell. Note the shell membrane (E), mammillary layer (M), and honeycomb layer (H). (F) Innermost (egg content side) layer of the shell membrane depicting the numerous blebs caused by the projecting mammillary knobs of the mammillary layer. (G) View of the transversely fractured organic layer in the nonopaque zone of a 2-day egg. Note the smooth fibers of organic matrix interspersed with numerous small calcite crystals. (H) View of the transversely fractured organic layer in the opaque central zone of a 56-day egg. Note the scarcity of small calcite crystals (see Fig. G) and the blebbing on the fibers of organic matrix. (I) High power view of the fibers in the shell membrane of a 3D-day egg. Note the interweaving of the fibers, the numerous blebs on the surfaces of the fibers, and the porous nature of the membrane. Fig. 9. Alligator mississippiensis. Scanning electron micrographs (SEM). (A) Surface and fractured edge of an IS-day eggshell. Note the smooth shell surface (5), the outer, densely
372 REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCODILIANS ment for the calcite crystals, but may be the templates for crystal growth. Beginning about the third week of incubation, the number of small calcite crystals in the organic layer beneath the opaque band decreases (compare Fig. 9G with 9H), and this process continues as the band expands. It appears that these crystals, in preference to the larger units in the mammillary layer, are mobilized for embryonic calcification. This process steadily weakens the calcified shell, and horizontal separation usually begins a few days before hatching, so that only the shell membrane and parts of the mammillary and organic layers surround the embryo, with the rest of the shell either cleaved off or intact (but weakened). 5. MAMMILLARY LAYER The mammillary layer (Figs. 7, 9E, and 9F) comprises numerous cones ("mammillary knobs") with their bases on the organic layer and their tips continuous with the shell membrane at the core of the knobs. The calcite crystals of the mammillary knobs are larger than those elsewhere and are arranged into regular prism-like subunits, which are clearly evident in radial sections of the shell (Fig. 9E). In tangential section, the mammillae appear as irregular hexagons. Pores (Fig. 7) occur wherever the irregular faces of one or more mammillae fail to contact each other in the horizontal plane as has been described for the avian egg (Tullett, 1975). Pores are most numerous in the central opaque region of the eggshell. Mammillary outlines are much easier to distinguish in the central opaque region than in the terminal nonopaque ends, where the shell membrane has flat areas of attachment. Shell membrane fibers attach to the central organic core of the mammillary knobs. 6. SHELL MEMBRANE The shell membrane consists of randomly oriented fibers (Fig. 7) that interweave extensively in a criss-cross pattern, enclosing air spaces (Fig. 91). The fibers are approximately 2 j..Lm in diameter and show the characteristic surface blebbing of glycosaminoglycans (Fig. 91). The innermost surface of the membrane has a thin, amorphous, nonfibrous, surface-limiting layer of unknown composition (Fig. 9F). Pores in the latter (Figs. 7 and 9F) are the innermost end of a complex system of spaces in the more superficial layers, which connect the contents of the egg (e.g., the chorioallantoic membrane) to the external environment and are probably important in respiration and water regulation. Contrary to some reports (Bigalke, 1931; Pooley, 1962, 1969a; Bellairs, 1969; Guibe, 1970), crocodilian eggs, unlike those of birds, do not have an air space (Reese, 1915a, 1931a; McIlhenny, 1935; Packard et al., 1977; Ferguson, 1982a). Abnormal air spaces may form in dehydrated eggs where shrinkage of the shell membrane cleaves it from the shell through the organic layer (Ferguson, 1982a,b; Ferguson and Joanen, 1983). (Also see Section I.G.) THE EGGSHELL AND SHELL MEMBRANES 7. EROSION CRATERS 373 Throughout incubation, the egg surface develops erosion craters, which appear as stepped concentric rings each corresponding to one crystal layer of the outer densely calcified zone (Figs. 7, 8, and 9C). They are caused by crystal dissolution, possibly caused by acidic metabolites of nesting microorganisms and/or by the hydration of expired carbon dioxide. These craters appear to be present in the eggshells of numerous species (Ferguson, unpublished data), including fossil crocodilians (Hirsch, 1984). Details on their mode of formation are summarized in Fig. 8 (Ferguson, 1981c, 1982a). Their development confers possible advantages to the embryo in terms of enhanced exchanges of respiratory gases and water vapor across the shell as well as the facilitation of hatching through eggshell weakening. The occurrence of extrinsic degradation is of importance to those incubating eggs in embryological, farming, management, or conservation projects. Normally eggs should be left in their natural dirty state and incubated completely surrounded by damp nesting media in order to facilitate the production of bacterial acids. However, clean eggs can be successfully incubated without nesting media by maintaining very high humidities (circa 90-100%). The high humidity ensures that all of the expired carbon dioxide produces carbonic acid, which degrades the shell. Presumably, this hydration, possibly in combination with mineral acids from the moist nest sand, forms the erosion craters in hole-nesting crocodilians (Ferguson, 1981c). The relative importance of humidity, or acids produced by microbial fermentation of nesting vegetation, in normal degradation of the eggs of mound nesters has not been adequately determined (Ferguson, 1981c, 1982a). However, embryos from eggs incubated at humidities below 90% without nesting media, frequently die (probably from asphyxia or the toxic effects of waste products), whereas the remaining living hatchlings usually have to be manually helped out of the tough eggshell (Ferguson, 1981c, 1982a). Further investigations are also required into which specific microorganisms are involved, whether they derive any benefit (e.g., calcium) from the process, the changing pH of the eggshell surface in the nest, which acids are involved, and how the system evolved. In other systems, various fungi penetrate and degrade calcareous substances such as mollusk shells (Kohlmeyer, 1969, 1972). Although thermophilic fungi have been isolated from alligator nests (Tansey, 1973), their role in eggshell degradation is unknown. 8. PORES Single, unbranched pores run vertically upwards from spaces between the mammillae to end at the shell surface (Figs. 7 and 90). They are more numerous in the central opaque region of the eggshell, and in freshly laid eggs, their openings are capped by an organic plug (Figs. 7 and 90). During incubation, the openings of the pores are widened by extrinsic
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