AbstractFractions of sperm homogenates from various animals (medaka, loach, toad, and rat) have been injected into unfertilized mature eggs of the medaka,Oryzias latipes, to test their cleavage‐initiating activity (CIA). The relationship between CIA and components of each fraction was also examined by observation with an electron microscope. It was found that there was high CIA in the fraction containing flagella or microtubules, but little CIA in the supernatant fluid without them. The CIA was closely related to the concentration of flagellum components and had no species‐specificity. However, further development after the blastula stage might be influenced by a species‐specific factor(s) of some flagellum compo