With reference to the distribution of the stroma in the germinal epithelium, it may be said in a general way that there is a special layer close to the surface of the ovary, which, after the formation of fresh ova has nearly ceased, completely isolates a superficial layer of the germinal epithelium from the deeper and major part of it. The superficial layer is frequently (but erroneously) regarded as constituting the whole of the germinal epithelium. The layer of stroma below the superficial epithelium forms in the mammalian ovary the tunica albuginea. As the follicles are formed. in the trabeculæ of germinal epithelium the stroma grows in around them, and forms for each one of them a special tunic. The adult ovaries differ in a corresponding manner to the embryonic genital ridges as to the presence of a core of stroma. The ovaries which are without such a core in the embryo, are also without it in the adult, and are formed of a double layer of tissue entirely derived from the germinal epithelium with its ingrowths of stroma, and composed, for the most part, of ova in all stages of development. In the case of the other ovaries there is a hilus of stroma--the zona vasculosa--internal to the egg-bearing region. In Mammalia, proportionately to the ovary, the zona vasculosa is at a maximum, and in Birds and Reptiles it is relatively far less developed. In these forms the germinal epithelium covers the whole surface of the ovary. In Elasmobranchii the structure of the ovary is somewhat different, owing to the presence in the ovarian ridge of a large quantity of a peculiar lymphatic tissue, which has no homologue in the other ovaries; and still more to the fact that the true germinal epithelium is in most forms entirely confined to the outer surface of the ovary, on which it forms a layer of thickened epithelium in the embryo (fig. 17), and of ovigerous tissue in the adult. In the ovary of Mammalia and Reptilia and possibly other forms there are present in the zona vasculosa during embryonic life cords of epithelial tissue derived from the Malpighian bodies; these cords have no function in the female, but in the male assist in forming the seminiferous tubules. In considering the development of the ova it is again convenient to distinguish between Amphioxus and the Craniata.