The invertebrates display several methods of asexual reproduction, such as fission and gemnule formation and budding (Hay, 1966). Planaria reproduce asexually only by fission.

A study was made of the postulated fission-inhibiting substance located in the cephalic region of Dugesia dorotocephala. Rearing decapitated planaria in four different concentrations (25, 50, 75 or 100 ug/ml) of head or whole body extract resulted in a positive correlation between the inhibition of fission rate and the concentration of extract. The difference in inhibitory activity between the two types of extracts was three percent at maximum. The inhibitory substance was partially inactivated at 60° C. There was a direct correlation between the length of time that the extract was heated and the inactivation of inhibitor. The active substance was precipitable with ammonium sulfate. Fractionation of whole body extracts with a Sephadex G-200 column showed the presence of three peaks. The material from all three peaks exhibited inhibitory activity, possibly indicating the presence of more than one inhibitory substance. Fractionation procedures with DEAE anion exchange columns inactivated the inhibitory substance(s). The mechanism of action of the inhibitory substance(s) is not known. The animals must be exposed to the extract within 24 hours after decapitation in order for maximum inhibition to be obtained.