The ultrastructure of the test of the tadpole larva ofCiona intestinalis

Summary The test of the tadpole larva ofCiona intestinalis consists of an amorphous background substance in which are long 4–7 nm fibrils. There is a narrow outer dense region of the test where both the background material and the test fibrils are more concentrated and orientated. Fibrils of similar dimensions are found in the epithelial cells below the test, and also in cells associated with the outside surface of the test. No such fibrils are found in the cells located within the test substance. There are some regions where the external cell membrane of the epithelial cells is indistinct and in these regions the fibrils of the test are continuous with the fibrils within the epithelial cells. No large or small vesicles opening from the epithelial cells into the test have been seen.This evidence has been interpreted as support for the tunicization theory of test formation. It is suggested that the cells within the test are the source of the polyphenols that are necessary for quinone tanning that gives rigidity to the test.The fibril-containing cells outside the test probably add fibres to the cuticular region of the test.I wish to thank ProfessorJ. Z. Young, F.R.S. for much advice and encouragment, also Dr.R. Bellairs for the use of electron microscope facilities and Mr.R. Moss and Mrs.J. Hamilton for skillful technical assistance.