The formation of the ovum is most simple in the type without yolk cells, which will for that reason be first considered (fig. 17 _A_). The germogen is constituted of a number of nuclei imbedded in a scanty cementing protoplasm. In the lower part of the germogen the nuclei are larger, and become separated off from the nucleated protoplasm above, as distinct cells with a thin layer of protoplasm round the germinal vesicle. These cells are the ova. As they pass down the egg-tube their protoplasm increases in bulk, and they become isolated by ingrowths of the epithelial cells the origin of which is still uncertain, which form round each ovum a special follicle, so that the egg-tube is filled by a single row of ova each in an epithelial follicle (fig. 17 _A_). The larger the ova the more columnar is the epithelium of the follicle. As the oviductal extremity of the egg-tube is approached the ova increase in size, and their protoplasm is more and more filled with yolk particles. In the lower part of the egg-tube the epithelium gives rise to a chorion. The epithelium around each ovum has been spoken of as forming a follicle, and it is implied that the epithelium round each ovum travels down the egg-tube with the ovum. It is however by no means clear from the observations of the majority of writers that this is the case, and in fact the epithelium is generally spoken of as if it were simply the epithelium of the egg-tube. In favour of the view here adopted the following considerations may be urged. Firstly, there is considerable evidence that the superficial layer of the germogen gives rise to the epithelial cells, simultaneously with the formation of the ova from the deeper layers. Secondly, the fact that the epithelium grows in between the separate ova appears to render it almost certain that this part of the epithelium must travel down the egg-tubes with the ova. Thirdly, the epithelium no doubt gives rise to the chorion, and considering the peculiar structure of the chorion, this seems possible only on the view that the epithelium travels down the egg-tube with the ova. Fourthly, when, or even before, the egg is laid the epithelium undergoes atrophy, and the remains of it have been compared to the corpora lutea. If the view about the epithelium here adopted is correct, the epithelium without doubt corresponds to the follicular epithelium of other ova, and has the same origin as the ova themselves. The ovaries with yolk cells differ in appearance from those without, mainly in each ovarian chamber of an egg-tube containing two elements, usually more or less distinctly separated. These two elements are (1) at the lower end of the chamber, the ovum, and (2) at the upper, large cells which gradually disappear as the ovum grows larger (fig. 17 _B_).