It is well known that sea urchin embryos treated with lithium chloride (LiC1) develop to abnormally into vegetalised embryos, in which differentiation of ectodermal cells is inhibited. When embryos of the sea urchins, Hemicentrotus pulcherrimus and Anthocidaris crassispina were treated with 20 mM LiC1 from the 8-cell stage to the corresponding early gastrula stage, they developed to vegetalised embryos with a large exogut 45 h after fertilisation. In these vegetalised embryos, high activity of alkaline phosphatase (AP) was detected histochemically at the end of the exogut where it is attached to the embryo body. High activity of AP is known to be detected specifically in the gut of sea urchin pluteus larvae by the same procedure as used in this study. Hence, we concluded that this part of the exogut is composed of the cells which develop into the cells of the gut in normal development.

When exogut isolated from vegetalised embryos was cultured in the extract obtained from eggs or embryos, the end composed of the cells in which high AP activity was detected, expanded during culture and formed a large spherical structure about 24 h after the initiation of culture. The minimum concentration of extract to cause expansion of isolated exogut was 5 × 103 egg or embryo equivalent/ml ASW (artificial seawater). The extract boiled at 95 °C for 1 h also caused expansion of isolated exogut at the same concentrations as non-boiled extract. On the other hand, the extract obtained from eggs or embryos by chloroform–methanol extraction did not cause any expansion of exogut, but the aqueous phase, heat-dried and dissolved in ASW, induced expansion of isolated exogut.