Peronella japonica, a direct developer, exhibits certain peculiar features during development, particularly heterochrony, a change in the relative timing of expression among tissues and organs. One of the important heterochronical changes in the species was found in the development of the amniotic cavity, a component of an adult rudiment. In indirect developers the amniotic cavity is formed on the left side of the larval body in the late pluteus stage. In P. japonica the organ is formed at the gastrula stage in the region located on the midline of the larval body.

In the present study, the ability of partial embryos isolated from 8- or 16-cell stage embryos of P. japonica to differentiate an amniotic cavity was investigated to assess the regulative potential of a direct developer.

The embryos were dissected at 8-cell stage with a glass needle to obtain half embryos. Some of the half embryos were further divided into four blastomeres to obtain mesomere pairs. Each half embryo and blastomere that did not form micromeres but divided equally during the next cleavage was identified as an animal cap and presumptive mesomere pair. Isolated animal caps and mesomere pairs were cultured, and differentiation of the amniotic cavity was examined at 24 and 48 h after fertilisation, when the organ in the normal embryos had already completed differentiation.