Ontogeny of Marine Turtle Gonads
201
membrane, and shell. The eggs are ready to be oviposited after about 9 days following
ovulation (Miller, 1985); however, the internesting interval is typically longer
(Miller, 1985; 1997). Embryonic development is arrested at middle gastrulation until
oviposition (stage 6, Miller, 1985).
At oviposition the blastodisc is composed of epiblast (presumptive ectoderm),
and hypoblast (presumptive endoderm), and the area between is filling with migrating
epiblastic cells (presumptive mesoderm) (Agassiz, 1857; Mitsukuri, 1894; Fujiwara,
1966; 1971). The dorsal expression of the chordamesodermal canal has the shape
of an anteriorly opening, wide crescent (Mitsukuri, 1896–98; Miller, 1985). The
canal has not broken through ventrally. During the early days of postovipositional
development, the three germinal layers in the area opaca spread laterally and peripherally over the yolk mass that is contained in the follicular yolk membrane to form
the extraembryonic splanchnopleure and the extraembryonic somatopleure (Agassiz,
1857; Mitsukuri, 1894; Fujiwara, 1966; 1971). These eventually give rise to the yolk
sac and the allantoic membranes, and the amnion and chorion, respectively. The
space between the yolk and the embryonic disk is filled by subgerminal fluid.
When oviposited into the nest chamber, the embryonic disk on the vitelline
(follicular) membrane may land in any position relative to gravity. Within moments
the vitelline membrane carrying the embryonic disk begins to rotate to the top pole
of the yolk via the liquefaction of the surrounding albumen and the pull of gravity
on the unevenly distributed, viscous yolk material contained within the vitelline
membrane.
Over the next few hours, the albumen liquefies above the embryonic disk and
passes through the margins of the embryonic area and vitelline membrane into the
subgerminal area. This causes the vitelline membrane to distort to become more
pear-shaped and causes the embryonic disk and yolk to rise toward the inner shell
membrane. Simultaneously, the oviducal fluid that filled the microscopic canals
among the aragonite crystals of the eggshell (Solomon and Baird, 1976; 1979;
Solomon and Watt, 1985; Chan and Solomon, 1989) drains by capillary action down
around the outer portion of the eggshell and/or inward to become part of the fluid
layer just within the inner shell membrane. This action opens the pathway for gas
exchange. The rising of the embryo on the distorted vitelline membrane reduces the
distance over which gas exchange occurs. Together, these actions facilitate embryonic respiration (via diffusion) before development and vascularization of the
extraembryonic membranes. Because the vitelline membrane is distorted and
stretched, movement of the egg may cause it to rupture and the embryo to die
(Limpus et al., 1979; Parmenter, 1980; Chan et al., 1985); the embryo remains subject
to movement-induced mortality until it has established the extraembryonic membranes, about 25 days into incubation (Parmenter, 1980).
After oviposition, the development of the embryo is a continuous process. Once
the egg has stabilized in the nest, the chordamesodermal canal breaks through
ventrally. The neural plate forms above the notochord; the headfold becomes obvious. Somites begin to form just behind the neural folds and continue to form in pairs
in a craniocaudal direction as the dorsal mesoderm subdivides into segments. Within
2.5 days at 30 ∞ C, the embryo reaches stage 10 (see Miller, 1985, for descriptions
of embryonic stages).
1123 book.book Page 201 Monday, November 11, 2002 11:11 AM
201
membrane, and shell. The eggs are ready to be oviposited after about 9 days following
ovulation (Miller, 1985); however, the internesting interval is typically longer
(Miller, 1985; 1997). Embryonic development is arrested at middle gastrulation until
oviposition (stage 6, Miller, 1985).
At oviposition the blastodisc is composed of epiblast (presumptive ectoderm),
and hypoblast (presumptive endoderm), and the area between is filling with migrating
epiblastic cells (presumptive mesoderm) (Agassiz, 1857; Mitsukuri, 1894; Fujiwara,
1966; 1971). The dorsal expression of the chordamesodermal canal has the shape
of an anteriorly opening, wide crescent (Mitsukuri, 1896–98; Miller, 1985). The
canal has not broken through ventrally. During the early days of postovipositional
development, the three germinal layers in the area opaca spread laterally and peripherally over the yolk mass that is contained in the follicular yolk membrane to form
the extraembryonic splanchnopleure and the extraembryonic somatopleure (Agassiz,
1857; Mitsukuri, 1894; Fujiwara, 1966; 1971). These eventually give rise to the yolk
sac and the allantoic membranes, and the amnion and chorion, respectively. The
space between the yolk and the embryonic disk is filled by subgerminal fluid.
When oviposited into the nest chamber, the embryonic disk on the vitelline
(follicular) membrane may land in any position relative to gravity. Within moments
the vitelline membrane carrying the embryonic disk begins to rotate to the top pole
of the yolk via the liquefaction of the surrounding albumen and the pull of gravity
on the unevenly distributed, viscous yolk material contained within the vitelline
membrane.
Over the next few hours, the albumen liquefies above the embryonic disk and
passes through the margins of the embryonic area and vitelline membrane into the
subgerminal area. This causes the vitelline membrane to distort to become more
pear-shaped and causes the embryonic disk and yolk to rise toward the inner shell
membrane. Simultaneously, the oviducal fluid that filled the microscopic canals
among the aragonite crystals of the eggshell (Solomon and Baird, 1976; 1979;
Solomon and Watt, 1985; Chan and Solomon, 1989) drains by capillary action down
around the outer portion of the eggshell and/or inward to become part of the fluid
layer just within the inner shell membrane. This action opens the pathway for gas
exchange. The rising of the embryo on the distorted vitelline membrane reduces the
distance over which gas exchange occurs. Together, these actions facilitate embryonic respiration (via diffusion) before development and vascularization of the
extraembryonic membranes. Because the vitelline membrane is distorted and
stretched, movement of the egg may cause it to rupture and the embryo to die
(Limpus et al., 1979; Parmenter, 1980; Chan et al., 1985); the embryo remains subject
to movement-induced mortality until it has established the extraembryonic membranes, about 25 days into incubation (Parmenter, 1980).
After oviposition, the development of the embryo is a continuous process. Once
the egg has stabilized in the nest, the chordamesodermal canal breaks through
ventrally. The neural plate forms above the notochord; the headfold becomes obvious. Somites begin to form just behind the neural folds and continue to form in pairs
in a craniocaudal direction as the dorsal mesoderm subdivides into segments. Within
2.5 days at 30 ∞ C, the embryo reaches stage 10 (see Miller, 1985, for descriptions
of embryonic stages).
1123 book.book Page 201 Monday, November 11, 2002 11:11 AM