In two asexual planarian strains (Dugesiu gonocephala and Dugesia tigrina), reproduction by transverse fission and regeneration is observed under normal conditions and upon decapitation and reserpine treatment. The pattern of asexual reproduction processes in planarians, as it results from our quantitative observations compared with the literature data, indicates that autotomy and regeneration processes have an extremely wide range of possible expressions. In such a heterogeneous situation it is still possible to suppose the existence of a negative cephalic control of fissiparity, similar to the ones involved in general tissue organization and in the regeneration of planarians.
The asexual reproduction that occurs in several planarian species consists essentially in a classical architomy, i.e., in transverse fission that divides the worm in anterior and posterior segments, which reconstruct the whole animal by caudal and cephalic regeneration, respectively (Hyman, '51; de Beauchamp, '61; Brdndsted, '69). The architomic mechanism of asexual reproduction differs from the paratomic mechanism, in which the organ regeneration precedes the parting of the new specimens (Wagner, 1890).
Fission and regeneration are related to several environmental conditions such as temperature, population density, feeding, and lighting (Ogukawa and Kawakatsu, '56a,b). Kenk ('37) and Benazzi ('74