The number of eggs in 23 clutches ranged from two to eight (mean, 4.7
A+- 1.37 [sigma]); clutches of four, five, and six eggs were most
common, occurring in 18 (78 per cent) instances. The tendency for
large females to lay more eggs than small females (Fig. 6) was not so
pronounced as that reported by Cagle (1950:38) for _Pseudemys
scripta_. The small size of _T. ornata_, in comparison with other
emyid turtles, seemingly limits the number of eggs that can be
accommodated internally. The number of eggs per clutch in T. carolina
[2 to 7, average 4.2, Allard (1935:331)], is nearly the same as that
of _T. ornata_.
[Illustration: FIG. 6. The relation of plastral length to
number of eggs laid by 21 females of _T. o. ornata_ from
eastern Kansas.]
Shells of the eggs are translucent and pinkish or yellowish when the
eggs are in the oviducts. After several days outside the oviducts the
shells become chalky-white and nearly opaque. Eggs incubated in the
laboratory retained the pinkish color somewhat longer than elsewhere
on their under-surfaces, which were in contact with moist cotton, but
eventually even this part of the shell became white. Infertile eggs
remained translucent and eventually became dark yellow, never becoming
white; they could be distinguished from fertile eggs on the basis of
color alone. Shells of infertile eggs became brittle and slimy after
several weeks.
The outer layer of the shell of a freshly laid egg is brittle and
cracks when the egg is dented. After a few days, when the eggs begin
to expand, the shell becomes flexible and has a leathery texture. The
shell is finely granulated but appears smooth to the unaided eye. The
granulations are approximately the same as those illustrated by
Agassiz (1857:Pl. 7, Fig. 18) for _T. carolina_.
Eggs are ellipsoidal. Data concerning size and weight (consisting of
mean, one standard deviation, and extremes, respectively) taken from
42 eggs (representing 9 clutches) within 24 hours after they were
laid, or dissected from oviducts, are as follows: length, 36.06 A+- 2.77
(31.3-40.9); width, 21.72 A+- 1.04 (20.0-26.3); and weight, 10.09 A+- 1.31
(8.0-14.3). There was a general tendency for smaller clutches to have
larger eggs; the largest and heaviest were in the smallest clutch (two
eggs) and the smallest were in the largest clutch (eight eggs). Risley
(1933:697) reported such a correlation in _Sternotherus odoratus_, as
did Allard (1935:331) in _T. carolina_. Measurements in the literature
of the size of eggs of _T. ornata_ suggest a width greater than that
stated above, probably because some eggs already had begun to expand
when measured.
Eggs of _T. ornata_ expand in the course of incubation, as do other
reptilian eggs with flexible shells, owing to absorption of water. In
the laboratory, 48 eggs increased by an average of approximately three
grams in weight and three millimeters in width over the entire period
of incubation; increase in width coincided with decrease in length.
Cotton in incubation dishes was kept moist enough so that some water
could be squeezed from it. When the cotton was constantly moist, eggs
showed a fairly steady expansion from the first week of incubation
until hatching. The process could be reversed by allowing the cotton
to dry. Eggs that were allowed to dry for a day or more became grossly
dented or collapsed. Eggs at the periphery of the incubation dish were
ordinarily more seriously affected by drying than were those at the
center or in the bottom of the dish. A generous re-wetting of
desiccated eggs and cotton caused the eggs to swell to their original
proportions within 24 hours. Recessions occurred, however, even in the
clutches that received the most nearly even amount of moisture.
Increases in weight and size seemed to reach a peak in the middle of
the incubation period and again immediately before hatching. Infertile
eggs expanded in the same manner as fertile eggs in the first week or
two of incubation, but thereafter gradually regressed in bulk or
failed to re-expand after temporary periods of dryness. Fertile eggs
that were in good condition had a characteristically turgid, springy
feel and could be bounced off a hard surface.
Temporary lack of moisture usually did not kill embryos; prolonged
dryness, combined with high temperatures, probably could not be
tolerated. Lynn and Ullrich (1950), by desiccating the eggs of
_Chrysemys picta_ and _Chelydra serpentina_, produced abnormalities in
the young ranging from slight irregularities of the shell to eyeless
monstrosities; eggs desiccated in the latter half of incubation
produced a higher percentage of abnormal young than eggs that were
desiccated earlier.
In 1956, three fertile eggs, from clutches that were at different
stages of incubation, were immersed in water for 48 hours. The eggs
rested on the bottom of the bowl in the same position in which they
had been placed in the incubation dishes; when turned, they returned
invariably to the original position. The embryos in two of the eggs
(one and 27 days old at the time of immersion) were still living ten
days after the eggs were removed from the water; the embryo in the
remaining egg (21 days old at the time of immersion) was dead. Eggs
immersed in water increased in size and weight at the same rate as
eggs in incubation dishes, indicating that absorption of water
probably operates on a threshold principle, the amount absorbed being
no more than normal even under wet conditions.
Natural nests usually are in well-drained areas, but water probably
stands in some nests for short periods after heavy rains. Provided the
nest cavity itself is not damaged, water in the nest is probably more
beneficial than harmful to the eggs; however, nests that are inundated
during floods probably have little chance of survival.