III. The embryology of Monotremata and Marsupialia. Part I

In Monotremata, in very young ova, a fine membrane exists between the single row of follicular cells and the substance of the ovum. This membrane, which I will callthe vitelline membrane , at first increases in thickness with the growth of the ovum, and through it pass numerous fine protoplasmic processes connecting the protoplasm of the follicular cells with that of the ovum, and serving to conduct food granules, which, appearing in the neighbourhood of the nuclei of the cells, travel thence to the ovum; food granules also appear in the neighbourhood of the germinal vesicle, and travel away from it: hence the horse-shoe shape of the yolk-mass as seen in section. The time during which food granules are thus passing from the follicular cells to the ovum maybe called “the yolk forming period."