Testes were preserved in each month from April to October. The
following description of spermatogenesis is based chiefly on material
collected in 1955, although testes were preserved also in 1954.
Comparison of material obtained in 1954 and 1955 revealed that
spermatogenesis began earlier and was more advanced on any given date
in 1955 than in 1954.
Testes of mature individuals are pale yellow and slightly oblong. The
epididymis is ordinarily dark brown or black and contrasts sharply
with the color of the testes. Size of testes was expressed as the
average length (greatest diameter) of both testes. Testes are smallest
in April, immediately after emergence from hibernation, and largest in
early September (Pl. 20, Figs. 3-4). They are nearly spherical when of
maximum size; increase in bulk, therefore, is relatively greater than
the increase in size shown in Figure 3. They increase in size from
April until early June, recede during most of June, and again increase
in size in July and August. They remain large from early September
until hibernation is begun, becoming only slightly smaller in late
September and October.
Increase in size following emergence from hibernation may be due in
part to proliferation of the sustentacular cytoplasm. Decrease in size
in early June is correlated with the end of the period of most active
mating; maximal size is coincident with the peak of the spermatogenic
cycle in early September.
[Illustration: FIG. 3. Seasonal fluctuations in size (average
greatest diameter) of testes in _T. o. ornata_ as determined
by examination of 40 specimens from eastern Kansas.]
Spermatogenesis (refer to Pl. 19, Figs. 1-5) begins in early May when
a few spermatogonia appear in the seminiferous tubules. The
histological appearance of testes preserved in April and May is much
the same. Nuclei of Sertoli cells, which outnumber the spermatogonia,
are evident at the periphery of the tubules and the clear cytoplasm of
the cells extends into and nearly fills the lumina. The few darkly
stained spermatids that are present in April are cells that probably
were produced in the previous summer. Sperm are present in small
groups within the sustentacular cytoplasm, but ordinarily are absent
in the lumina.
Primary spermatocytes appear in the tubules from mid-May to early
June. By mid-May there are practically no sperm at any place in the
tubules. The sustentacular cytoplasm has a less compact arrangement in
late May than in April.
Spermatogenesis is well under way by mid-June; at this time, two or
three distinct layers of primary and secondary spermatocytes are
present and these cells outnumber the Sertoli cells. The lumina are
filled with cellular detritus and are no longer bordered by a clear
ring of sustentacular cytoplasm. No sperm are present.
Spermatids appear in late June and a few of them undergo metamorphosis
in early July; by mid-July, spermatids and secondary spermatocytes are
the dominant cells in the seminiferous tubules, although
spermatogonia are still active.
By late August, clusters of sperm and metamorphosing spermatids
surround the Sertoli cells; large numbers of sperm as well as sloughed
cells representing various spermatogenic stages are present in the
lumina. Secondary spermatocytes are still evident near the periphery
of the tubules but they are much less numerous than spermatids. The
germinal epithelium is still semiactive and small groups of primary
spermatocytes are present in nearly all of the tubules.
The spermatogenic cycle is completed in the latter half of October
when most of the spermatozoa pass into the epididymides. A few
spermatozoa and spermatids remain in the seminiferous tubules during
hibernation. Although no testicular material was obtained from
hibernating turtles, comparisons of sections made in October and April
show that the germinal epithelium remains inactive from autumn until
spring. Possibly some spermiogenesis takes place in the early phases
of hibernation or in the period in late autumn when turtles are
intermittently active. It is uncertain whether the reorganization of
the sustentacular cytoplasm occurs in autumn, in spring, or in the
course of hibernation.
The seminiferous tubules of immature males are small, lack lumina, and
contain a few large but inactive spermatogonia (Pl. 19, Fig. 6). The
testes of specimens that were nearly mature contained primary and
secondary spermatocytes but lacked lumina; it was thought that such
individuals would have matured in the following summer and bred in the
following autumn.
Mature sperm were found in epididymides at all times of the year but
were most numerous in spring and autumn, the period between
spermatogenic cycles (Pl. 19, Fig. 7). Sperm expelled from the
epididymides in autumn matings are seemingly replaced by others from
the seminiferous tubules; the epididymides become much smaller when
their supply of sperm is nearly exhausted after spring mating.
Risley (1938:304) found the testes of the common musk turtle,
_Sternotherus odoratus_, to be largest in August and smallest in early
May. Recession of testes in spring was coincident with the period of
active breeding; increase in size, later in the season, corresponded
to increasing spermatogenic activity and enlargement of seminiferous
tubules. Altland (1951:600-603) found the spermatogenic cycle of
_Terrapene carolina_ to be nearly like that of _Sternotherus
odoratus_. Fox (1952) found that testes of garter snakes (_Thamnophis
sirtalis_ and _T. elegans_) in California reached a peak of
spermatogenic activity in midsummer, regressed in the latter half of
the summer, and were inactive in winter.
The spermatogenic cycle of _T. ornata_ as here reported, differs in no
important respect from those of _Thamnophis_, _Sternotherus odoratus_,
or _Terrapene carolina_, except that in _T. ornata_ the cycle begins
and ends somewhat later in the season of activity. In most of the
lizards that have been studied (Fox, 1952:492-3), spermatogenesis
reaches a peak in spring (more or less coincident with the mating
period and with ovulation) and the germinal epithelium remains active
in winter. _Sternotherus_, _Terrapene_, and _Thamnophis_ are alike in
completing spermatogenesis late in the season and storing spermatozoa,
in the seminiferous tubules or in the epididymides, during
hibernation.
It is noteworthy that, in the turtles and snakes mentioned above,
sperm produced in autumn are used to fertilize eggs laid in the
following year, and mating [with the exception of _Thamnophis
elegans_, (Fox, 1956)] occurs in both spring and autumn. It is not
definitely known in any of these instances, whether sperm resulting
from autumn or spring inseminations (or both) fertilize the eggs.
Risley (1933:693) found motile sperm in the oviducts of female
_Sternotherus odoratus_ that had recently emerged from hibernation;
he believed that spring mating, although it commonly occurred, was not
necessary to fertilize eggs. Disadvantages, if any, of completing
spermatogenesis well in advance of ovulation seem to be at least
partly counteracted by two annual mating periods or by mating
throughout the season of activity.