In 1966, american biologist lynn margulis proposed the theory of endosymbiosis, or the idea that mitochondria are the descendants of symbiotic, aerobic eubacteria. what does the theory of endosymbiosis explain?

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.

i believe the answer is snake.