Planarians, a group of flatworms, exhibit an extraordinary ability to regenerate their entire bodies from small fragments. This regenerative capacity relies on clonogenic neoblasts, which are pluripotent stem cells actively dividing and differentiating to rebuild lost tissues.

What Are Clonogenic Neoblasts?

• Neoblasts are the only proliferative cells in planarians, capable of self-renewal and differentiation into all cell types.

• They migrate to wound sites, proliferate extensively, and form a blastema—a mass of undifferentiated cells that serves as the source for new tissue growth.

Epimorphosis in Planarians

• The mode of regeneration involving proliferating stem cells forming a blastema followed by differentiation is called epimorphosis.

• In planarians, regeneration after amputation requires the activation, migration, and proliferation of neoblasts, fitting the hallmark features of epimorphosis.

Why Other Options Are Less Appropriate

• Morphallaxis (Option 2) primarily involves tissue remodeling without significant cell proliferation, unlike planarian regeneration.

• Stem cell-mediated regeneration (Option 3) is broadly correct but does not specify the involvement of blastema formation and tissue growth. Epimorphosis is a more precise term.

• Compensatory regeneration (Option 4) refers to growth by enlargement of existing cells or tissues, not blastema formation from stem cells.

Summary

The regeneration of planarians by clonogenic neoblasts exemplifies epimorphosis, a stem cell-driven process involving new tissue formation from a proliferative blastema, restoring complex body structures after injury.

Final Answer:
(1) epimorphosis