The Submicroscopic Structure of the Drosophila Egg

Abstract

A study has been made with the electron microscope of the structure of the growing oocyte and associated nurse and follicle cells in Drosophila melanogaster. The fully grown oocyte has a cortical plasm which is thrown into deep folds, at the base of which are small delta granules. Immediately below this is a zone of periplasm which contains: well-developed mitochondria; some ergastoplasmic elements, probably mainly tubular, though sometimes taking the form of concentric oval sheets; many alpha granules which when well fixed are irregularly star-shaped and are probably lipoidal in composition; a number of stacks of annulate lamellae. Yolk granules, which may be built of a few tens of smaller particles, are found mainly at deeper levels, and so are the electron-light and probably fluid beta granules and the rather darker gamma granules. Epsilon granules, with a definite membrane and vacuolated interior, occur mainly in the posterior end of the egg and later in the pole cells. The vitelline membrane, which in earlier stages has a spongy structure, begins to appear between oocyte and follicle cells earlier than below the nurse cells. The appearance of small vesicles in the ooplasm beneath the developing membrane suggests that it is secreted mainly by the oocyte. There is very complex folding of the membranes between oocyte and follicle cells and still more between oocyte and nurse cells. Attention is drawn to: (i) the great enlargement in the area of the nurse-cell nuclear membranes, (ii) the striking differences in appearance between the nuclear membranes in the nurse cells and follicle cells, (iii) the formation of elaborately organized ergastoplasmic structures in the cytoplasm of the degenerating nurse cells, (iv) the large number and orderly arrangement of the elements in the stacks of annulate lamellae, which is held to suggest a process of in situ synthesis.