Fine Structure of Pigment Inclusions in the Test Cells of the Ovary of Styela

In the tunicate ovary, both germ cells and follicle cells are generally believed to arise from the ger-minal epithelium (cf., Seeliger (3) and Tucker (5)). The young germ cell moves internally from the periphery of the ovary and, together with several epithelial cells, constitutes the primary follicle. Initially, the germ cell is surrounded only by a flattened layer of epithelium. The cells of the epithelial layer, however, soon divide so as to form two layers of follicle cells, an inner and outer layer, and another follicle cell which becomes compressed in the periphery of the developing oocyte. The latter is referred to as the test cell and has no counterpart in any vertebrate ovary thus far examined. The test cell becomes separated from the follicle epithelium after formation of the vitel-line membrane (chorion). A diagram illustrating these relationships is shown in Fig. I. Alternative views concerning the origin of the oocyte-follicle cell complex in ascideans can be obtained by consulting Van Beneden and Julin (6), Spek (4), and Knaben (2). Recently, studies were made on the fine structure of the ovary of Molgula, with special reference to the changes occurring in the oocyte, follicle cells, and test cells during oogenesis (1). Schematic diagram showing relationship of test cell (T), inner follicle cell (IF), outer follicle cell (OF), and vitelline membrane (C) surrounding the developing oocyte of Styela. In subsequent observations on the ovary of Styela, certain differences were noted especially with regard to the presence of pigmented inclusions with a unique fine structure in the test cell and are reported in this note.