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

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366 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS
o 8
TO T1 T3 T5
T7 87 T30 T52
Fig. 6. Alligator mississippiensis. Diagram depicting the development of the opaque band in
eggs viewed in transmitted light. T, top of egg; B, bottom of egg; numbers, days after the time
the egg was laid.
shell after day 7 (Figs. SA to C, G to J, and 6). This extension in width is not
matched by a uniform expansion in length, so that the opaque band tapers
from a maximum length at the top surface of the shell to a minimum length
on its bottom surface (Figs. 5 and 6). The embryo always lies beneath the
upper expanded part of the fusiform opaque ring (Figs. 5B, E, F, G-J, and
6). The opaque band then expands rapidly in length so that it extends over
approximately 60% of the top surface and 50% of the lower surface of the
shell between days 8 and 30 (Figs. 5B to 0, G to J, and 6). The top part of
the band reaches the ends of the egg around day 40 and the bottom part
around day 50; thereafter the egg is completely opaque and so appears
unbanded (Figs. 50, J, and 6). The opaque part of the egg never transmits
light as well as the adjacent translucent regions and is clearly demonstrated
as a dark zone if the eggs are transiIIuminated (Figs. 5G to J, and 6).
Eggshell banding represents a useful, external indicator for estimating
the age of alligator eggs (Ferguson, 1982a) up to approximately day 50;
subsequently the eggs are completely opaque. The normal expansion of
the opaque band may be retarded in eggs containing malformed or developmentally
retarded embryos. Usually, the expansion of the "minimum
band length" at the bottom of the egg is the most sensitive indicator
(Ferguson, 1982a, b). Eggs containing embryos that have died but
that are not infected show an arrest of band development at the stage of
embryonic death. This may be followed by regression of the band as the
embryo autolyzes. Infected eggs have the opaque bands vaguely defined;
opaque blotchy patches may appear and disappear all over the eggshell,
usually in an erratic fashion. Infertile eggs never become banded and never
become infected (unless the eggshell or eggshell membrane is damaged or
THE EGGSHELL AND SHELL MEMBRANES
367
malformed), but remain uniformly translucent white (egg contents yellow)
throughout incubation. They can easily be distinguished from normal,
completely opaque, post day 50 eggs by transillumination (Figs. 5G to J,
and 6). Eggs laid at advanced stages of embryonic development, for example,
by stressed females (see Section II.C), are either banded at the time
of laying or become so very rapidly. Band expansion is also more rapid, but
the embryos usually die, at which time the formation ceases. Failure to
distinguish between unbanded infertile eggs and either those in which the
embryo has died early and become autolyzed or post day 50 normal unbanded
eggs has given rise to erroneous accounts of the relationship between
banding and fertility (Deitz and Hines, 1980; Tryon, 1980).
That the appearance and regular development of the opaque band indicate
normal, healthy development of the embryo is of value not only in
aging eggs but also in identifying abnormal development and monitoring
teratogenic experiments (Ferguson, 1981a, 1982b). The relationship between
the development of this band and eggshell hydration, albumen
metabolism, the formation of the extraembryonic membranes, structural
changes in the eggshell and changes in the composition of the shell membrane
is discussed in Sections III.C through E and IV.
C. Structure
1. GENERAL
The combined thickness of the eggshell and shell membrane of Alligator
mississippiensis is approximately 0.5 to 1.0 mm and consists of five layers
(Fig. 7). From the surface inwards, there is an outer, densely calcified layer
(100-200-f.Lm thick), a honeycomb layer (300-400-f.Lm thick), an organic
layer (8-12-f.Lm thick), a mammillary layer (20-29-f.Lm thick), and the shell
membrane (150-250-f.Lm thick). Pores run from the egg surface through the
calcified layers and end in the shell membrane (Fig. 7). The outer openings
of the pores become modified and numerous erosion craters develop as
incubation progresses (Figs. 7 and 8). The different layers of the eggshell
consist of varying amounts of calcite crystals and organic matrix. The
freshly laid egg is coated with slimy oviducal secretions, which dry and
then disappear.
2. OUTER DENSELY CALCIFIED LAYER
At low magnification (Fig. 9A), the egg surface appears smooth, but at
higher magnification (Fig. 9B), the granular nature of the numerous calcite
crystals is evident. There is no detectable organic matrix between the
rhombohedral crystals in this layer; their form, size, and orientation are
shown in Figs. 8 and 9, and further details are given in Ferguson (1981c,
1982a). The calcite crystals are stacked on their ends (or faces) in this layer.