The proportions of the shell change somewhat in the first few weeks of
life. At hatching the shell may be misshapen as a result of
confinement in the egg. Early changes in proportions of the shell
result from expansion--widening and, to a lesser degree, lengthening
of the carapace--immediately after hatching. Subsequent retraction or
rupture of the yolk sac and closure of the navel are accompanied by a
decrease in height of shell and slight, further widening of the
carapace.
The yolk sac retracts mainly between the time when the egg shell is
first punctured and the time when the turtle actually emerges from the
shell. When hatching is completed, the yolk sac usually protrudes no
more than two millimeters, but in some individuals it is large and
retracts slowly over a period of several days.
One individual began hatching on November 11 and was completely out of
the egg shell next day; the yolk sac was 15 millimeters in diameter,
protruded six millimeters from the umbilical opening, and hindered
the hatchling's movements. The sac broke two days later, smearing the
bottom of the turtle's dish with semifluid yolk. The hatchling then
became more active. Twenty-six days later the turtle was still in good
condition and its navel was nearly closed. A turtle that hatched with
a large yolk sac in a natural nest possibly would benefit, through
increased ease of mobility, if the yolk sac ruptured.
A recently hatched turtle was found at the Reservation in October,
1954, and was kept in a moist terrarium in the laboratory where it
died the following May. The turtle was sluggish and ate only five or
six mealworms while in captivity; no growth was detectable on the
laminae of the shell. Autopsy revealed a vestige of the retracted yolk
sac, approximately one millimeter in diameter, on the small intestine.
The navel ("umbilical scar") of captive hatchlings ordinarily closed
by the end of the second month but in three instances remained open
more than 99 days. The position of the navel is marked by a
crescent-shaped crease, on the abdominal lamina, that persists until
the plastron is worn down in later years (Pl. 24, Fig. 1).
[Illustration: FIG. 7. A hatchling of _T. o. ornata_ (A-- 2) that
still retains the caruncle ("egg tooth"). A distinct boss will
remain on the maxillary beak after the caruncle is shed.]
The caruncle ("egg tooth") (Fig. 7) remains attached to the horny
maxillary beak for a variable length of time; 93 per cent of the live
hatchlings kept in the laboratory retained the caruncle on the tenth
day, 71 per cent on the twentieth day, and only 10 per cent on the
thirtieth day of life. Few individuals retained the caruncle when they
entered hibernation late in November, and none retained it upon
emergence from hibernation. Activities in the first few days or weeks
of life influence the length of time that the caruncle is retained;
turtles that begin feeding soon after hatching probably lose the
caruncle more quickly than do those that remain quiescent. The
caruncles of some laboratory specimens became worn before finally
dropping off. Almost every caruncle present after 50 days could be
flicked off easily with a probe or fingernail. The initiation of
growth of the horny maxillary beak probably causes some loosening of
the caruncle. The caruncle may aid hatchlings in escaping from the
nest.
After the caruncle falls off, a distinct boss remains, marking its
former place on the horny beak (Pl. 25, Fig. 1); this boss is
gradually obliterated over a period of weeks by wear and by
differential growth, and is seldom visible in turtles that have begun
their first full year of growth. The "first full year of growth" is
here considered to be the period of growth beginning in the spring
after hatching.