204
The Biology of Sea Turtles, Vol. II
cells are interspersed among the cells of the cortex. The medullary area shows some
differentiation of cells to form primitive medullary (sex) cords, but for the most part,
these regress and the area remains a thick mass of undifferentiated cells that will be
penetrated by blood vessels.
In presumptive males, the testis results from the simultaneous regression of the
cortex and the differentiation of seminiferous tubules in the medulla. In the cortex,
cells regress to become a flattened epithelium, whereas cells in the medulla differentiate to form hollow sex cords that twist and anastomose to form the seminiferous
tubules, which eventually connect to the rete testis and efferent tubules. The primordial germ cells migrate to reside among the cells lining the tubules. Mesenchyme
cells condense to form the tunica albuginea that is situated between the cortex and
the medulla of the gonad; it eventually becomes a thin, vascular, connective tissue
sheath surrounding the testis and covered by a thin epithelium.
In marine turtles, the sex of the turtle is determined in the thermosensitive
period (stages 22–27, Miller, 1985) of incubation rather than at fertilization ( C.
caretta : Yntema and Mrosovsky, 1980; 1982; Wibbels et al., 1991; C. mydas:
Miller and Limpus, 1981; Lepidochelys olivacea : Mohanty-Hejmadi and Dimond,
1986; McCoy et al., 1983; Merchant-Larios and Villalpando, 1990; MerchantLarios et al., 1989; 1997; L. kempii : Shaver et al., 1988; Eretmochelys imbricata :
Wibbels et al., 1999a; Natator depressus : Hewavisenthi and Parmenter, 2000; D.
coriacea : Desvages et al., 1993; Rimblot et al., 1985). Although much remains to
be elucidated about the impact on embryonic development of variations in environmental temperature, gas concentration, and moisture availability during incubation (Ackerman, 1997), the clearest picture of the impact of incubation temperature on the differentiation of the gonads of marine turtles occurs toward the
extremes of embryonic tolerance (approximately 23–33 ∞ C). In general, female
hatchlings result from eggs incubated above 30 ∞ C and males result from eggs
incubated below 28 ∞ C. The theoretical point at which a 1:1 sex ratio would be
produced has been termed the pivotal temperature (Mrosovsky and Yntema, 1980),
the critical temperature (Pieau, 1973), and the threshold temperature (Bull, 1980).
The theoretical temperature can be estimated in the manner of lethal dose 50
(LD 50 ) calculations to provide an estimate of the 95% confidence limits (Limpus
et al., 1983), which is useful for comparing among populations. From both the
population function and conservation management points of view, the differences
among populations are important.
7.2.4 G ENITAL D UCTS
The genital ducts provide the means by which the sex cells (sperm, ova) are
passed to the exterior of the body. Within the testis, as the medulla differentiates,
the seminiferous tubules become connected with the tubules of the rete testis
that in turn connect to the tubules of the anterior mesonephros (termed vasa
efferentia). The vasa efferentia combine with the anterior portion of the vas
deferens to become the epididymis. The remainder of the mesonephric duct
(Wolffian duct) becomes the vasa deferentia and, although originally derived
1123 book.book Page 204 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
cells are interspersed among the cells of the cortex. The medullary area shows some
differentiation of cells to form primitive medullary (sex) cords, but for the most part,
these regress and the area remains a thick mass of undifferentiated cells that will be
penetrated by blood vessels.
In presumptive males, the testis results from the simultaneous regression of the
cortex and the differentiation of seminiferous tubules in the medulla. In the cortex,
cells regress to become a flattened epithelium, whereas cells in the medulla differentiate to form hollow sex cords that twist and anastomose to form the seminiferous
tubules, which eventually connect to the rete testis and efferent tubules. The primordial germ cells migrate to reside among the cells lining the tubules. Mesenchyme
cells condense to form the tunica albuginea that is situated between the cortex and
the medulla of the gonad; it eventually becomes a thin, vascular, connective tissue
sheath surrounding the testis and covered by a thin epithelium.
In marine turtles, the sex of the turtle is determined in the thermosensitive
period (stages 22–27, Miller, 1985) of incubation rather than at fertilization ( C.
caretta : Yntema and Mrosovsky, 1980; 1982; Wibbels et al., 1991; C. mydas:
Miller and Limpus, 1981; Lepidochelys olivacea : Mohanty-Hejmadi and Dimond,
1986; McCoy et al., 1983; Merchant-Larios and Villalpando, 1990; MerchantLarios et al., 1989; 1997; L. kempii : Shaver et al., 1988; Eretmochelys imbricata :
Wibbels et al., 1999a; Natator depressus : Hewavisenthi and Parmenter, 2000; D.
coriacea : Desvages et al., 1993; Rimblot et al., 1985). Although much remains to
be elucidated about the impact on embryonic development of variations in environmental temperature, gas concentration, and moisture availability during incubation (Ackerman, 1997), the clearest picture of the impact of incubation temperature on the differentiation of the gonads of marine turtles occurs toward the
extremes of embryonic tolerance (approximately 23–33 ∞ C). In general, female
hatchlings result from eggs incubated above 30 ∞ C and males result from eggs
incubated below 28 ∞ C. The theoretical point at which a 1:1 sex ratio would be
produced has been termed the pivotal temperature (Mrosovsky and Yntema, 1980),
the critical temperature (Pieau, 1973), and the threshold temperature (Bull, 1980).
The theoretical temperature can be estimated in the manner of lethal dose 50
(LD 50 ) calculations to provide an estimate of the 95% confidence limits (Limpus
et al., 1983), which is useful for comparing among populations. From both the
population function and conservation management points of view, the differences
among populations are important.
7.2.4 G ENITAL D UCTS
The genital ducts provide the means by which the sex cells (sperm, ova) are
passed to the exterior of the body. Within the testis, as the medulla differentiates,
the seminiferous tubules become connected with the tubules of the rete testis
that in turn connect to the tubules of the anterior mesonephros (termed vasa
efferentia). The vasa efferentia combine with the anterior portion of the vas
deferens to become the epididymis. The remainder of the mesonephric duct
(Wolffian duct) becomes the vasa deferentia and, although originally derived
1123 book.book Page 204 Monday, November 11, 2002 11:11 AM