In Holothuria polii, the periesophageal ring is an important organ supplying spherule cells after stimulation with foreign material. In animals injected with formalinized sheep erythrocytes, in fact, a depletion of spherule cells is observed in the periesophageal ring, whereas in the connective tissue, in the external epithelium and around the antigen- injected site, small, transparent cells can be visualized. It is supposed that the latter are stem cells of spherule cells
