The stroma grows into the germinal epithelium while it is still formed of rounded indifferent cells, and divides it into trabeculæ as described above. At a later period a number of the cells in the deeper layer of the epithelium, as well as certain cells in the superficial part, become primitive germinal cells, while the remainder of the cells become smaller and are destined to form the follicle cells. The most conspicuous primitive germinal cells are situated in the superficial layer of epithelium; and the primitive germinal cells in the deeper layers of the germinal epithelium are not nearly so marked as in most Craniata, so that it is difficult in some cases to be sure of their destination till their nucleus commences to undergo its characteristic metamorphosis. The change of the primitive ova into permanent ova takes place in the same manner in Mammals as in Elasmobranchii, except that the fusion of the primitive ova into polynuclear masses is much rarer. The formation of the at first quite simple follicles takes place while the ova are still aggregated in large masses; and the first follicles are formed in the innermost part of the germinal epithelium. Soon after their formation the follicles become isolated by connective-tissue growths. _Post-embryonic development of the ova._ The ova of the Vertebrata differ greatly in size and structure. The differences in size depend upon the quantity of the food-yolk. In the Amphioxus and Mammalia, in which the ova are smallest, the comparatively insignificant amount of food-yolk is distributed uniformly through the ovum. A larger quantity of it is present in the ova of Amphibia, Marsipobranchii and Teleostei, and it attains an immense development in the ova of Elasmobranchii, Reptilia, and Aves. The food-yolk originates from a differentiation of the protoplasm of the egg. It arises as a number of small highly refracting particles in a stratum slightly below the surface.