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Crocodilians, marine turtles, some terrestrial turtles and lizards undergo environmental sex determination. The important factor for sex determination in these species is the incubation temperature of the eggs as this determines the development of the gonads towards testes or ovaries. In crocodiles a higher temperature is male determining whilst in turtles a higher temperature is female determining.

Crocodilians, marine turtles, some terrestrial turtles and lizards undergo environmental sex determination. The important factor for sex determination in these species is the incubation temperature of the eggs as this determines the development of the gonads towards testes or ovaries. In crocodiles a higher temperature is male determining whilst in turtles a higher temperature is female determining.

==Sexual Differentiation of the Brain==

==Sexual Differentiation of the Brain==

In the male testosterone is secreted into the blood and is able to cross the [[Blood_Brain_Barrier_-_Anatomy_%26_Physiology|blood brain barrier]]. Once across this barrier the testosterone is converted into '''estradiol''' and '''dihydrotestosterone'''. Although it is counter-intuitive, esradiol actually causes masculinisation of the male brain.

In the male testosterone is secreted from the fetal testes into the blood and is able to cross the [[Blood_Brain_Barrier_-_Anatomy_%26_Physiology|blood brain barrier]] as it does not bind to alpha-fetoprotein. Once across this barrier the testosterone is converted into '''estradiol''' and '''dihydrotestosterone''' via the enzyme '''aromatase'''. Although it is counter-intuitive, esradiol actually causes masculinisation of the male brain. This brain masculinisation process results in the prevention of the development of a surge centre.

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In the female '''alpha-fetoprotein''' binds to estradiol resulting in this complex being unable to cross the [[Blood_Brain_Barrier_-_Anatomy_%26_Physiology|blood brain barrier]]. This mechanism protects the female brain from the masculinising effects of estradiol.

In the female '''alpha-fetoprotein''' binds to estradiol which has been secreted by the fetal ovary. This binding results in the complex being unable to cross the [[Blood_Brain_Barrier_-_Anatomy_%26_Physiology|blood brain barrier]]. This mechanism protects the female brain from the masculinising effects of estradiol and allows the development of a 'feminising' '''surge centre''' at the point of the rostral part of the [[Midbrain_-_Anatomy_%26_Physiology|mesencephalon]].

==EMBRYOLOGICAL DEVELOPMENT OF THE GENITAL TRACT==

==EMBRYOLOGICAL DEVELOPMENT OF THE GENITAL TRACT==