For some modern reptiles typical of a live birth, some remained in transition: the embryo develops inside their eggs, but the egg is in the body of the mother. But turtles do not even think about the live birth and continue to lay eggs, and it turns out it is they have a good reason.
Scientists from Monash University have found that the turtles are able to influence the rate of development of the embryo. Sows can reduce oxygen content in the oviduct using a special mucous substance resulting embryo development is stopped at a certain point, however temporarily "freezes". In this case, the condition of the fetus’ oxygen overlap "does not tell: he stays healthy, and upon the occurrence of a favorable environment continues to develop normally on.
Turtles are very accurately pick a time when you need to stop and re-start the development of the embryo. If you try to hold the fetus in anoxic conditions a little more time, he will be killed or else it will develop numerous diseases.
As noted by biologists, thanks to a trick with oxygen turtles can choose the most appropriate time for the birth of offspring. Under adverse conditions, they are able to delay the development of a baby and wait until conditions improved a little bit of life. When the situation is normalized around, turtle lays eggs and embryo development continues. If the turtles decide to live birth or yaytsezhivorozhdenie, such a stunt would not have passed.
On the other hand, this ability can be a threat to turtles in the face of unexpected environmental changes. Any new substance can bring down their program management concentration of oxygen in the fallopian tubes, causing germs can be "frozen" on the wrong stage. This, of course, affect the population.