Algae were supplied continuously to four-armed pluteus larvae of the sea urchin Hemicentrotus pulcherrimus until the adult rudiment reached stage g, which is the initial stage of rudiment formation (Chino et al., 1994) After stage g, one group of larvae was reared without the addition of algae for comparison of the development of the adult rudiment with that in larvae given algae. Three days later, only 9.1 ± 0.3% of larvae without algae had reached stage j, while 76.8 ± 0.6% of larvae with algae had reached a stage beyond j, which indicates the formation of the complete adult rudiment.

When larvae at rudiment stage g were reared in a medium supplemented with T4 or its derivatives, such as T3, 3,3′,5′-L-triiodothyronine (rT3) or triiodothyropropionine (Tp3), in place of algae, the adult rudiment developed in a dose-dependent manner. T4 was the most effective and induced formation of the adult rudiment in more than 70% of specimens at 1 nM and in almost 100% at 100 nM. T3 was one-tenth as effective as T4. Other derivatives were still less effective. On the other hand, casein, ovalbumin, tyrosine and a mixture of 20 amino acids had no effect on the development of larvae and adult rudiments, suggesting that they were not available as nutrients and sources of thyroid hormones.