Reproductive Biology and Embryology of the ... - Seaturtle.org

STAGES OF EMBRYONIC OEVELOPMENT [AFTER EGG LAYING)
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STAGE 6 7 8/9 10
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STAGE 12 13 14 15
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STAGE 20 21 22 25
and between species in association with definitive scale and scute patterns.
Pigment is also present ventrally and on all limb elements.
Limbs. Scales are present on the proximal and distal elements. The nail
tips of the feet have a slight distal elevation (Fig. 23).
Jaws. Sensory papillae are present along the lateral jaw margins and
scales are evident on the gular skin (Fig. 24J).
Caruncle. The caruncle is located on a smooth white base (Fig. 26£).
Brain. The midbrain is visible as a white bulge at the back of the
cranium because the overlying skin is poorly pigmented and the osseous
cranial roof is incomplete.
Pericardial Sac. The pericardial sac is three-fourths withdrawn into the
body cavity.
Palate. An extensive plexus of blood vessels and sensory papillae has
formed (Fig. 25G).
Species Differences. In C. johnsoni and C. porosus the scales along the jaw
margins are triangular with the apex of the triangle toward the jaw margins,
giving the latter a serrated appearance. The external genitalia are
larger and more differentiated than in A. mississippiensis.
St;ags 24 (Figs. 21, 23, and 24KJ
Coloration. Pigmentation is now denser so that embryos appear blacker
in color. Various patterns occur both within and between species.
Limbs. The nails on the hand also have elevations at their tips (Fig. 23)
and these elevations are starting to form the curves at the tip of the nails.
Brain. The midbrain, enclosed by bone, is overlain by pigmented skin.
Pericardial Sac. The pericardial sac is fully withdrawn into the body
cavity and the ventral thoracic wall is closing in the midline.
Yolk. A large volume of yolk lies outside the body and the ventral
umbilical area is large.
Scales and Scutes. The scales and scutes are now very evident as elevations
all over the embryo (Fig. 24K).
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Fig. 23. The typical (diagnostic) appearances of crocodilian right fore and left hind limbs
(including hands, feet and nails) at various stages of development. Stages 6 to 11 are views
from the dorsal aspect of the embryo and depict the projection of the limb anlage from the
flank. Stages 12 to 17 are lateral views of the sides of the embryo and depict the proximal and
distal elements of the limb anlage. Stages 18 to 28 are views of the hands, feet, and nails. Up to
Stage 17, no structures except the digital mesodermal condensations (at Stages 16 and 17) are
visible macroscopically in the limbs, thereafter anlage for the limb and digit cartilages, for the
nails and for the interdigital webbing are evident, hence the change in diagram style at Stage
18. Based on observation of Alligator mississippiensis, Crocodylus porosus, and C. johnsoni, although
some (Stages 18 to 28) drawings are adapted from Voeltzkow's (1899) work on C.
niloticus. AER, Apical ectodermal ridge; 0, mesodermal condensations for the digits; DC, digit
cartilages; N, nail anlage.