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

406 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS
Palate. The secondary palatal shelves are one-fourth closed at the beginning
of Stage 18 and three-fourths closed at the end (Figs. 25B and C).
This stage can be accurately subdivided by specifying the extent of secondary
palate closure (Figs. 25B and C, and 28C-E). The upper jaw margin is
straighter and less hooked than previous.
Eye. The margins of the upper eyelid anlage extend over the superior
rim of the iris forming a distinct groove between the eyelids and the eye,
into which small instruments can be passed.
Scales, Scutes, etc. Dorsal scalation is now marked.
PericardiaI Sac. The bulge of the transparent pericardial sac is starting to
be submerged into the ventral thoracic wall.
Stage 19 (Figs. 20, 23, 250, 26B, 34E and F]
Eye. Upper and lower eyelids are distinct. The anterior nictitating
membrane anlage is discernible in the anterior corner of the eye (Fig. 26B).
Caruncle. Two elevations of the caruncle have approximated each other
at the tip of the snout (Fig. 26B), but the tissue between them is thin,
appearing transparent under incident illumination.
Lower Jaw. The lower jaw lies behind the anterior margin of the upper
jaw. Consequently, if the premaxillary bulges are large, the mouth opens
(as in Fig. 24H). The tongue and floor of the mouth contents sag beneath
the margins of the lower jaw.
Limbs. Interdigital clefting has commenced producing slight marginal
notches particularly in the foot plates (Fig. 23).
Palate. The palate is almost completely closed (Fig. 250).
Coloration. White flecks, representing underlying ossifications, are obvious
on the margins of the upper and lower jaws and around the ears.
External Genitalia. The end of the external genitalia has developed a
globular swelling.
Denticles. There are eight to nine denticles visible on each side of the
lower jaw. Henceforth, many mandibular denticles appear and disappear
so that their numbers are too variable to be used in staging.
Species Differences. In C. johnsoni and C. porosus, the nostrils are elevated
on a nasal disk. The lateral jaw margins have distinct notches where
the primary and secondary palates closed (as in Figs. 24H and I); these later
accommodate the large fourth dentary teeth.
Stage 20 (Figs. 20, 23, 24H, I, 25E, and 26C]
Limbs. Nail anlagen develop rapidly in a specific sequence early in this
stage (Fig. 23). They appear first on the most medial digit of the foot, then
on the neighboring two digits, then on the most medial digit of the hand
and finally on the neighboring two medial hand digits. Consequently, nail
anlagen are present on the most medial three digits of both the hands and
STAGES OF EMBRYONIC DEVELOPMENT [AFTER EGG LAYING)
407
feet, despite the fact that the total number of digits varies between the two
(Fig. 21). Interdigital clefting now extends along approximately one-fourth
the length of the digits (Fig. 23). The outer two digits of the hand and the
outer digit of the foot never develop nails.
Caruncle. The caruncle is now a solid structure due to consolidation of
the region between the two initial swellings (Fig. 26C).
Lower Jaw. The lower jaw is in its adult relationship with the upper jaw
(Figs. 24H and I).
External Genitalia. The external genital primordium is now pointed
with a distinct elevation of its tip.
Palate. The palate is completely closed but the basihyal valve is not yet
present (Fig. 25E).
PericardiaI Sac. The pericardial sac is now one-fourth withdrawn into
the ventral body cavity.
Coloration. White flecks of ossification are present along the margins of
the upper and lower jaws, around the external auditory meatus, and in the
proximal and distal elements of the limbs.
Scales and Scutes. Scale formation is marked dorsally and scutes are
beginning to appear in the neck region behind the skull.
Stage 21 (Figs. 20, 23, 25F, 260]
Limbs. Interdigital clefting extends three-fourths of the way along the
digits. Phalanges can be distinguished in the digits (Fig. 23).
Scales and Scutes. Scales are now visible on the ventral body wall as well
as dorsally on the snout, neck, body, and tail. The dorsal neck scutes are
clearly defined.
Caruncle. The caruncle is a solid mass on the snout tip, but the tissue
around the base of the caruncle is not differentiated from the other snout
scales (Fig. 260).
Palate. The superior basihyal valve flap is present at the posterior margin
of the palate (and the inferior flap at the base of the tongue) and a
plexus of palatal blood vessels is conspicuous (Fig. 25F).
Pericardial Sac. The pericardial sac is one-half withdrawn into the body
cavity.
External Nares. Elevations for the constrictor nares muscles are evident.
Eye. A white ring in the iris surrounds the outline of the lens of the eye
and is overlapped by both upper a.nd lower eyelids.
Stage 22 (Figs. 21, 23, 251]
Coloration. Pigmentation is first visible on the margins of the upper
jaw, along the ventral aspect of the flank, and on the proximal and distal
elements of the limbs, but there is little or no dorsal pigmentation.