The mode of conversion of the germinal cells into ova is somewhat diverse. Before the change takes place the germinal cells frequently multiply by division. The change itself usually involves a considerable enlargement of the germinal cell, and generally a change in the character of the germinal vesicle, which in most young ova (fig. 2) is very large as compared to the body of the ovum. The most complicated history of this kind is that of the ovum of the Craniata. (_Vide_ pp. 56, 57.) [FIG. 2. OVUM OF CARMARINA (GERYONIA) HASTATA. (Copied from Haeckel.) _gd._ Body of ovum. _gv._ Germinal vesicle. _gm._ Germinal spot.] The ovum in its young condition is obviously nothing but a simple cell; and such it remains till the period when it attains maturity. Nevertheless the changes which it undergoes in the course of its growth are of a very peculiar kind, and, consisting as they do in many instances of the absorption of other cells, have led various biologists to hold that the ovum is a compound structure. It becomes therefore necessary to consider the processes by which the growth and nutrition of the ovum is effected before dealing with the structure of the ovum at all periods of its history. [FIG. 3. FEMALE GONOPHORE OF TUBULARIA MESEMBRYANTHEMUM. CONTAINING ONE LARGE OVUM (_ov_) AND A NUMBER OF GERMINAL CELLS (_g.c._). _ep._ Epiblast (Ectoderm). _hy._ Hypoblast (Entoderm). _ov._ Ovum. _g.c._ Germinal cells.] The ovum is of course nourished like every other cell by the nutritive fluids in which it is surrounded, and special provisions are made for this, in that the ovary is very frequently placed in contiguity with vascular channels. But in addition to such nutrition a further nutrition, the details of which are given in the special part of this chapter, is provided for in the germinal cells which do not become ova. In the simplest case, as in many Hydrozoa (fig. 3), the germinal cells which do become ova are assimilated by the ovum much in the manner of an Amoeba. In other cases the ovum becomes invested by a special layer of cells, which then constitutes what is known as a follicle. The cells which form the follicle are often germinal cells, _e.g._ Holothuria, Insecta (fig. 17), Vertebrata (fig. 19). In other cases they seem rather to be adjoining connective-tissue or epithelioid cells, though it is sometimes difficult to draw the line between such cells and germinal cells. Examples of follicles formed of ordinary connective-tissue cells, are supplied by Asterias, Bonellia (fig. 16), Cephalopoda (fig. 14), etc. A membrane enclosing the ovum without a lining of cells, as in many Arachnida, _vide_ p. 51, has no true analogy with a follicle and does not deserve the same name.