Eggs were examined by transmitted light in the course of incubation.
At the time of laying (or removal from oviducts) no embryonic
structures were discernible even in eggs that had been retained in the
oviducts of captive females some weeks past the normal time of laying;
a colorless blastodisc could be seen if eggs were opened. Embryonic
structures first became visible at eight to ten days of incubation; at
this time vascularization of the blastodisc was evident and the eyes
appeared as dark spots. Heart beats were observed in most embryos by
the fifteenth day but were evident in a few as early as the tenth day.
The pulse of a fifteen-day-old embryo averaged 72 beats per minute at
a temperature of 30 degrees. Embryos at fifteen days, measured in a
straight line from cephalic flexure to posteriormost portion of body,
were approximately nine to ten millimeters long and at 22 days were 14
millimeters long. At approximately 35 days the eggs became dark red;
embryonic structures were discernible thereafter only in eggs that had
embryos situated at one end, close to the shell.
Incubation periods for 49 eggs (representing 12 clutches) kept in the
laboratory ranged from 56 to 127 days, depending on the temperature of
the air during the incubation period. In 1955, eggs were kept at my
home in Lawrence where air temperatures were uncomfortably hot in
summer and fluctuations of 20 degrees (Fahrenheit) or more in a
24-hour period were common. The following summer eggs were kept in my
office at the Museum where temperatures were but slightly cooler than
in my home and subject also to wide variation. In 1957 this part of
the Museum was air-conditioned and kept at approximately 75 degrees.
The greater lengths of incubation periods at lower temperatures are
shown in Table 1. Risley (1933:698) found the incubation period of
_Sternotherus odoratus_ to be longer at lower temperatures;
corresponding observations were made by Allard (1935:332) and Driver
(1946:173) on the eggs of _Terrapene carolina_. Cagle (1950:40) and
Cunningham (1939) found no distinct differences in length of
incubation period for eggs of _Pseudemys scripta_ and _Malaclemys
terrapin_, respectively, at different temperatures within the range
tolerated by the eggs.
Most nests observed in the field were in open situations where they
would receive the direct rays of the sun for at least part of the day;
the shorter average incubation periods (59 and 70 days, respectively),
observed in 1955 and 1956, therefore, more nearly reflect the time of
incubation under natural conditions than does the excessively long
period (125 days at 75 degrees) observed in 1957 under cooler, more
nearly even temperatures.
TABLE 1.--The Relationship of Temperature and Duration of Incubation
Period as Determined from Laboratory Studies of 49 Eggs of
_T. ornata_.
=============+===================+=========+========+=================
Average | Period of | Number | Number |
daily | incubation (Days) | of | of | Remarks
temperature |--------+----------|clutches | eggs |
(Fahrenheit) | Mean | Range | | |
-------------+--------+----------+---------+--------+-----------------
| | | | | Wide daily
91 | 59 | 56-64 | 6 | 24 | fluctuations in
| | | | | temperature
-------------+--------+----------+---------+--------+-----------------
| | | | | Wide daily
82 | 70 | 67-73 | 4 | 21 | fluctuations in
| | | | | temperature
-------------+--------+----------+---------+--------+-----------------
| | | | | Temperature
75 | 125 | 124-127 | 2 | 4 | thermostatically
| | | | | controlled
-------------+--------+----------+---------+--------+-----------------
Sixty-five days seems to be a realistic estimate of a typical
incubation period under natural conditions; eggs laid in mid-June
would hatch by mid-August. Even in years when summer temperatures are
much cooler than normal, eggs probably hatch by the end of October.
Hatchlings or eggs would have a poor chance of surviving a winter in
nests on exposed cut-banks or in other unprotected situations.
Overwintering in the nest, hatchlings might survive more often than
eggs, since hatchlings could burrow into the walls and floor of the
nest cavity. Unsuitable environmental conditions that delay the
nesting season and retard the rate of embryonic development may, in
some years, be important limiting factors on populations of ornate box
turtles.
In areas where _T. ornata_ and _T. carolina_ are sympatric (for
example, in Illinois, Kansas, and Missouri) the two species occupy
different habitats, _ornata_ preferring open grassland and _carolina_
wooded situations. Under natural conditions, the average incubation
periods of these two species can be expected to differ, _T. carolina_
having a somewhat longer period due to lower temperatures in nests
that are shaded. In the light of these speculations, the remark of
Cahn (1937:102)--that _T. ornata_ nested later in the season (in
Illinois) and compensated for this by having a shorter incubation
period--is understandable.
The range of temperatures tolerated by developing eggs probably varies
with the stage of embryonic development. When temperatures in the
laboratory were 102 to 107 degrees Fahrenheit for approximately eight
hours, due to a defect in a thermostat, the young in two eggs of _T.
ornata_, that had begun to hatch on the previous day, were killed, as
were the nearly full-term embryos in a number of eggs of _T. carolina_
(southern Mississippi) kept in the same container. A five-day-old
hatchling of _T. ornata_, kept in the same container, survived the
high temperatures with no apparent ill effects. Cagle (1950:41) found
that eggs of _Pseudemys scripta_ could not withstand temperatures of
10 degrees for two weeks nor would they survive if incubated at 40
degrees. Cunningham (1939) reported that eggs of _Malaclemys terrapin_
could not survive prolonged exposure to temperatures of 35 to 40.6
degrees but tolerated temporary exposure to temperatures as high as
46 degrees.
In the summer of 1955, a clutch of three eggs, all of which contained
nearly full-term embryos, was placed in a refrigerator for 48 hours.
The temperature in the refrigerator was maintained at approximately
4.5 degrees; maximum and minimum temperatures for the 48 hour period
were 2.8 and 9.5 degrees, respectively. When the eggs were removed
from the refrigerator they showed gains in weight and increases in
size comparable to eggs, containing embryos of the same age, used as
controls. The experimental eggs began to hatch two days after they
were removed to normal temperatures--approximately 24 hours later than
the controls.
In the late stages of incubation, the outer layer of the shell becomes
brittle and is covered with a mosaic of fine cracks or is raised into
small welts. Several days before hatching, movements of the embryo
disturb the surface of the shell and cause the outer layer to crumble
away, especially where the head and forequarters of the embryo lie
against the shell. Some embryos could be seen spasmodically thrusting
the head and neck dorsally against the shell.
The role of the caruncle in opening the shell seems to vary among
different species of turtles. Cagle (1950:41) reported that it was
used only occasionally by _Pseudemys scripta_; Allard (1935:332)
thought that it was not used by _Terrapene carolina_; and, the
observations of Booth (1958:262) and Grant (1936:228) indicate that
embryos of _Gopherus agassizi_ use the caruncle at least in the
initial rupturing of the shell.
In the three instances in which hatching was closely observed in _T.
ornata_, the caruncle made the initial opening in the shell; claws of
the forefeet may have torn shells in other hatchings that were not so
closely observed. In all observed instances, the shell was first
opened at a point opposite the anterior end of the embryo. The initial
opening had the appearance of a three-cornered tear. A quantity of
albuminous fluid oozed from eggs as soon as the shells were punctured.
The initial tear is enlarged by lateral movements of the front feet,
and later the hind feet reach forward and lengthen the tear farther
posteriorly. In many instances a tear develops on each side and the
egg has the appearance of being cleft longitudinally. The young turtle
emerges from the anterior end of the shell or backs out of the shell
through a lateral tear.
The process of hatching, from rupture of shell to completion of
emergence, extended over three to four days in the laboratory. Many
hatchlings from time to time crawled back into the shell over a period
of several days after hatching was completed. In a clutch of eggs kept
in a pail of earth, by William R. Brecheisen, eight days elapsed
between onset of hatching and appearance of the first hatchling at the
surface.
A nest in an outdoor pen at the Reservation was discovered in early
October. The cap had been recently perforated and the hatchlings had
escaped. One of them, judged to be approximately two weeks old, was
found in a burrow nearby. The cavity of the nest appeared to have been
enlarged by the young. The eggs were probably laid in early July.
Emergence of young from the nest had been delayed for a time after
hatching, until rain softened the ground in late September and early
October.