AN ovary of an adult female Drosophila melanogaster consists of a parallel cluster of ovarioles, each of which is differentiated into an anterior germarium and a posterior vitellarium (see KOCH, SMITH, and KING 1967, their Figure 1), The vitellarium is composed of a series of interconnected egg chambers which lie in single file. Each chamber is in a more advanced developmental stage than the one anterior to it, and each contains an oocyte and 15 nurse cells surrounded by a monolayer of follicle cells (ibid., Figure 2). The egg and its fifteen nurse cells are fourth generation descendants of a single germarial cell called a cystoblast. The cells formed by the mitotic activity of a cystoblast are called cystocytes, and it is within region 1 of the germarium that such mitoses occur (ibid., Figure 2). The cystocytes generated from a single germarial cystoblast form a branching chain of cells. The 16 cells are connected by 15 canals, each surrounded by a ring rich in protein (ibi: d., Figure 6). The cystocytes can be characterized by the number of ring canals each contains. Two cystocytes (designated le and 2e) are interconnected and each possesses four ring canals; two cells (3e and 4e) contain three canals each, four cells (5-8e) contain two ring canals each, and eight cells (9-16e) have but one canal each (ibid., Figure 7). Cells le and 2e undergo a different type of nuclear differentiation from cells 3-16e. Since only these two cells enter meiotic prophase, they have been named pro-oocytes, while the remaining fourteen are called pro-nurse cells. The production of fourth generation cystocytes and their differentiation into pro-oocytes and pro-nurse cells begins during the pupal stage, and continues throughout adult life (KING, AGGAR-WAL and AGGARWAL 1968: l. In each 16-cell cluster, however, one of the two prooocytes eventually switches to the nurse cell developmental pathway (BROWN and KING 1964).

Pro-oocytes may be readily differentiated from pro-nurse cells at the electron microscopic level by their nuclear morphology (KOCH, SMITH and KING 1967). Ribbon-like synaptonemal complexes are seen only in pro-oocyte nuclei (KING, AGGARWAL and AGGARWAL, their Figure 13). In the nucleus of the pro-oocyte