The function of the follicle cells appears to be, to elaborate nutriment for the growth of the ovum. The follicle cells are not as a rule directly absorbed into the body of the ovum, though in some instances, as in Sepia (_vide_ p. 40), they are eventually assimilated in this way. In many cases some of the germinal cells form a follicle, while other germinal cells form a mass within the follicle destined eventually to be used as pabulum. Insects supply the best known examples of this, but Piscicola, Bonellia (?) may also be cited as examples of the same character. In the Craniata (pp. 56-58) some of the germinal cells which advance a certain distance on the road towards becoming ova, are eventually used as pabulum, before the formation of the follicle; while other germinal cells form at a later period the follicular epithelium. A peculiar case is that of the Platyelminthes (fig. 9), where a kind of follicle is constituted by the cells of a specially differentiated part of the ovary, known as the yolk-gland. The cells of this follicle may either remain distinct, and continue to surround the ovum after its development has commenced, and so be used as food by the embryo; or they may secrete yolk particles, which enter directly into the protoplasm of the ovum. For further variations in the mode of nutrition the reader is referred to the special part of this chapter. Suffice it to say that none of the known modes of nutrition indicate that the ovum becomes a compound body any more than the fact of an Amoeba feeding on another Amoeba would imply that the first Amoeba ceased thereby to be a unicellular organism. The constitution of the ovum may be considered under three heads:-- (1) The body of the ovum. (2) The nucleus or germinal vesicle. (3) The investing membranes. The body of the ovum. The essential constituent of the body of the ovum is an active living protoplasm. As a rule there are present certain extraneous matters in addition, which have not the vital properties of protoplasm. The most important of these is known as food-yolk, which appears to be generally composed of an albuminoid matter.