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402 REPRODUCTIVE SIOLOGY AND EMSRYOLOGY OF CROCODILIANS STAGES OF EMSRYONIC DEVELOPMENT tAFTER EGG LAYING) 403 Facial Processes. The medial and lateral nasal processes are distinct elevations on either side of the nasal pits. The maxillary processes extend forward as far as the caudal junction of the medial and lateral nasal processes, and delimits a distinct groove beneath the eye. Limbs. The hindlimb bud is fan-shaped with a distinct apical ectodermal ridge (Fig. 23). It extends out further from the body wall than the forelimb bud. Tail. The tail is coiled through four 90 0 turns. Gut and Abdominal Organs. The mesonephros and liver are clearly visible through the lateral body walls. Stege 11 [Figs. 1 g end 23] Facial Processes. The nasal pit slit is starting to form between the medial and lateral nasal processes. The club-shaped maxillary processes extend to the junction of the medial and lateral nasal processes and is continuous with the lateral nasal process. Limbs. Both forelimb and hindlimb buds extend out caudally from the body wall, and both have distinct apical ectodermal ridges (Fig. 23). The forelimb has a distinct constriction demarcating the proximal and distal elements, but this constriction is less marked in the hindlimb. Gut and Abdominal Organs. A loop of midgut tube is visible at the level of the umbilicus. Branchial Arches. The 2nd branchial arch continues to overgrow the 3rd, and arches 4 and 5 are starting to submerge. The 1st branchial cleft is immediately ventral to the otocyst. Eye. Distinct black pigment present in the iris. Extra-embryonic Membranes. The chorioallantois extends two-thirds the way around the breadth of the shell membrane. Stage 12 [Figs. 1 g, 23, and 27H] Branchial Arches. The sinusoidal 1st branchial cleft lies above the otocyst and its margins show condensations for the auricular hillocks (Fig. 27H). The merged conglomerate of arches 1 and 2 is growing caudally and has overgrown arch 3 to reach the junction between the 3rd and 4th arches. This conglomerate forms the base of the lower jaw, which extends as far forward as the middle of the lens of the eye. Arches 4 and 5 are small but visible. The branchial sinus is patent. Facial Processes. The club-shaped maxillary process extends forward as a large shelf of tissue beneath the eye. The nasal pit slits are deepening as the medial and lateral nasal processes enlarge. There is a distinct notch and furrow in the midline of the face between the medial nasal processes of each side. Limbs. The forelimb, which is beginning to bend in the region of constriction for the proximal and distal elements, lies closer to the flank of the embryo. The elongated hindlimb shows little differentiation into proximal and distal elements, and although there is still a distinct apical ectodermal ridge, foot plate formation is just discernible (Fig. 23). Stsge 13 [Figs. 1 g, 23, 24A-C, 27H, 34C end 0] Facial Processes. The nasal pit slits are very distinct (Figs. 24A and B). The prominent maxillary processes are continuous with the lateral nasal processes (Figs. 24A-C). Limbs. The forelimb is now distinctly bent towards the pericardium. The distal hindlimb is flattened and enlarged into a footplate primordium (Fig. 23). Branchial Arches. Arch 3 is almost completely overgrown by arch 2, which now reaches the pericardium. Arches 4 and 5 are difficult to see. The branchial sinus has closed. The anterior margin of the lower jaw has grown forward from the merged conglomerate of arches 1 and 2. The 1st branchial cleft is now more horizontally oriented and is hereafter referred to as the external auditory meatus. The upper ear flap is sinusoidal with a midline bulge formed by the merging of the auricular hillocks (Fig. 27H). A groove runs craniocaudally along the basal (dorsal) aspects of the branchial arches and lower jaw. Extra-embryonic Membranes. The chorioallantois now extends as a ring around the inner circumference of the central eggshell membrane. Stege 14 [Figs. 1 g, 23, 240, 27F end H] Facial Processes. The nasal pit slit has closed due to the merging of the medial nasal, lateral nasal, and maxillary processes (Figs. 240 and 27F). The medial nasal processes are prominent and have anterodorsal external bulges (Figs. 240 and 27F). These will later grow forward to displace the external nares dorsally. The medial nasal processes also have two anterior intraoral bulges signifying the onset of primary palate development (Figs. 240 and 27F). The maxillary processes have sinusoidal intraoral margins signifying the onset of secondary palate development. The internal nares are distinct. Limbs. The foot and hand plates are distinct; the former is advanced over the latter (Fig. 23). Branchial Arches. The 2nd branchial arch has overgrown the 3rd, 4th, and 5th, and this merged conglomerate together with the 1st arch forms the base of the lower jaw and the neck. A craniocaudal groove is still present along the dorsal margins of the merged 1st and 2nd arches. Lower Jaw. The lower jaw extends one-fourth the way beneath the upper jaw. It is broad and round in A. mississippiensis but more pointed in C. johnsoni.
404 REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCOOILIANS STAGES OF EMBRYONIC OEVELOPMENT [AFTER EGG LAYING] 405 Ear. The upper ear flap is overgrowing the external ear opening. White opacities, internal condensations for ear development, are evident in the region below the ear opening. Denticles. One denticle is present on each side of the developing primary palate (merged medial nasal processes) margins and one on each side of the midline of the lower jaw. Flexures and Rotation. The embryonic face rests on the large bulge of the thorax (pericardial sac). Gut and Abdominal Viscera. A large loop of gut herniates through the narrow umbilical stalk and touches the underlying yolk. The abdominal viscera (e.g., liver and mesonephros) are visible through the body walls. Tail. This is markedly coiled, and the tip has a terminal kink. External Genitalia. In A. mississippiensis, the tubercle is still small, but in C. porosus and C. johnsoni it is much larger. Embryonic Reflexes. Contralateral withdrawal reflexes occur. Stage 15 [Figs. 20, 23, 24E, and 2711 Lower Jaw. This is one-third to one-half the length of the upper jaw (Fig. 24E). Dorsal to the lower jaw complex, there are three furrows and surface elevations, which appear to be dorsal representations of the branchial arches, which have now coalesced ventrally. Denticles. Two denticles are present on the anterior upper jaw margins of each side, one on the margin of the primary palate, the other on the maxillary process behind the closure zone. As before, one mandibular denticle is present on each side of the midline. Eye. Dark black stria in the iris radiate out from around the lens. The anlage for the upper eyelid is an elevated rim of tissue above each eye. Limbs. Distinct proximal and distal regions and hand and foot plates (but no digit rays) are present on both the forelimb and hindlimb (Fig. 23). Facial Processes. The anterodorsal (coalesced) bulges of the medial nasal processes have displaced the external nares dorsally (Fig. 24E). The primary palate has formed but still retains two anterior bulges (Fig. 24E). There is a distinct hollow in the face beneath the anterior one-third of the eye. The tectoseptal process is evident in the roof of the primitive oronasal cavity. Stage 1 6 [Figs. 20 and 231 Limbs. Faint digital condensations are present in the footplate but not in the handplate (Fig. 23). Lower Jaw. This is now two-thirds the length of the upper jaw. Denticles. There are four on the upper jaw margins of each side, two on the primary palate (that closest to the midline may be difficult to see), and two on the maxillary process. Two denticles lie on each side of the mandible. Facial Processes. The upper jaw is hook-shaped, being moulded around the pericardial bulge. Small secondary palatal shelves are present immediately behind the posterolateral margins of the primary palate. Caruncle. Two tiny, widely spaced thickenings are just discernible on the anterior tip of the snout. Stage 17 [Figs. 20, 23, 24F, G, 25A, 2SA and B1 Limbs. Mesodermal condensations for the five forelimb and four hindlimb digits are present (Fig. 23). Lower Jaw. The lower jaw lies behind the primary palate bulges (Figs. 24F and G). The furrows and elevations dorsal to the lower jaw have disappeared. Denticles. At this and later stages, the numbers of denticles discernible macroscopically in the upper jaw varies as some appear and others disappear; consequently, upper jaw denticle number is of little further diagnostic value. Contrarywise, seven denticles occur on each side of the lower jaw; three are distinct, three less distinct, and one is disappearing and very difficult to see. Caruncle. Two small elevations, widely separated, have appeared on the anterior tip of the snout (similar to Fig. 26A). Palate. The secondary palatal shelves are distinct. They are closest to each other anteriorly behind the primary palate, but do not contact (Figs. 25A, 28A and B). The tectoseptal processes are closing in the posterior half of the oronasal cavity. Scales, Scutes, etc. The somite pattern is distinct particularly on the dorsal aspects of the body and tail. This heralds the onset of scale differentiation. Anlagen for the abdominal ribs are visible. Flexures and Rotation. The head is extended off the bulge of the pericardial sac due to elongation of the neck. Ear. The external ear flap is distinct as the adult external ear form is established. Stage 1 S [Figs. 20, 23, 25B, C, 26A, and 2SC-F1 Limbs. The digit rays on the hand and foot plates are now distinct cartilaginous condensations. The hand and foot plates have a crenellated appearance (Fig. 23) caused by the straight margins of the interdigital tissues. Lower Jaw. The lower jaw lies beneath the anterior primary palate bulges, and it no longer rests on the pericardial sac. Denticles. At the start of this stage, six denticles are visible on each side of the mandible, and at the end, eight are discernible. The posterior denticles are difficult to see. Caruncle. Two widely spaced, white blebs on the anterior tip of the snout (Fig. 25A).
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