The present paper is an attempt to interpret, in terms of the cell-metabolism, the morphological changes exhibited by the actively secreting cell under natural conditions. For this purpose I have selected the ovarian egg. In no other cell are the structural changes more evident or their apparent correlation with the cell-activity more striking. The definite limits to the period of secretive activity and the fact that the products of secretion are stored up in the cell enable us to compare the appearances presented by the cell-structures at varying and easily recognisable phases of the cell-activity. Further, we can compare eggs in which a considerable quantity of yolk is formed unaided with those which form but little or have it partially formed for them. Finally, we are able in many cases to follow the gradual divergence from a common stock and structure of nutritive cells and definitive ova. Not only are we thus provided with exceptionally favourable opportunities for a comparative study of the egg under very different, but perfectly definite, physiological conditions, but the structural changes accompanying these conditions—the growth and varying form of the germinal vesicle, the behaviour of the chromatin and nucleolus, the appearance of the yolk nucleus and of yolk formation—all these constitute a wealth of morphological evidence as to the relative significance of the various cell-structures in the cell-metabolism. That this evidence has not as yet sufficed to enable us to come to any agreement as to even the most general physiological significance of the cell-structures in the cell-metabolism is, I believe, due not so much to insufficiency of data as to the manner in which these data have been studied; the behaviour of individual cell-structures is considered apart from the metabolism of the cell as a whole, and their changes in form are explained by the assumption of inconceivable structural complexity, while obvious physical factors are ignored.