The ultimate purpose is to understand "continuity of life". Life has been continued for billions of years without any gaps between generations. In vertebrates, differentiation of the testis and ovary from fertilized eggs, differentiation of sperms and eggs in the gonads, fertilization has been repeated in the continuity of life. I consider that ‘reproduction’ is the most important for "continuity of life". Thus, I have been investigated mechanisms regarding reproduction such as differentiation, development, and functions of gonads. I expect to understand the mystery of "continuity of life" in the future.