In Amphioxus the germinal epithelium which constitutes the essential part of the ovary is divided into a number of distinct segments: in the Craniata no such division is observable. In young examples of Amphioxus the generative organs are in an indifferent condition, and the two sexes cannot be distinguished. They form isolated horse-shoe shaped masses of cells, which occupy a position at the base of the myotomes, in the intervals between the successive segments; and extend from the hinder end of the respiratory sack to the abdominal pore. They are situated in the proper body cavity, and are surrounded by the peritoneal membrane. Each generative mass is at first solid, and is formed of an outer layer of more flattened cells and an inner mass of large rounded or polygonal cells. In its interior there appears at a somewhat later period a central cavity. After the cavity has appeared the sexes can be distinguished by the different behaviour of the cells. In all the Craniata, the ovary forms a paired ridge (unless single by abortion or fusion) attached by a mesentery to the dorsal wall of a more or less extended region of the abdominal cavity. This ridge is at first identical in the two sexes, and arises at an early period of embryonic life. It is essentially formed of a thickening of the peritoneal epithelium, and in Osseous Fish, Ganoids (?) and Amphibia the ovary remains during embryonic life nearly in this condition, though a small prominence of the adjacent stroma also becomes formed. In other Craniata the ridge, though at first in this condition, very soon becomes much more prominent, and is formed of a central core of stroma enclosed in the germinal epithelium (fig. 18). [FIG. 18. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (_po_) LYING IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.] The thickened germinal epithelium gives rise (in the case of the female) to the ova and the follicular epithelium. Whether the genital ridge is provided with a core of stroma or no, the germinal epithelium is always in contact on one side with the stroma, from which it is at first separated by a well-marked boundary line; but after a certain time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epithelium, and break it up into a sponge-like structure formed of trabeculæ of germinal epithelium interpenetrated by vascular strands of stroma. The trabeculæ of the germinal epithelium form the egg-tubes of Pflüger.