Note 5. To the Subsection “2.1. Yang and Yin Is Another Dual Characteristic”
Traditionally, it had been considered that single-cell organisms (bacteria) were sexless; but in 1946, J. Lederberg and E. L. Tatum demonstrated that even bacteria engage in sexual reproduction.
Concerning the sex of bacteria and paramecia, see, for example, Koichi Hiwatashi, The Search for the Origin of the Sex (in Japanese) (Tokyo: Iwanami Shoten, 1986).