The ovum at the time of impregnation has the form of a large spherical mass, similar to the yolk of a bird's egg, but without a vitelline membrane[18]. The greater part of it is formed of peculiar oval spherules of food-yolk, held together by a protoplasmic network. The protoplasm is especially concentrated in a small lens-shaped area, known as the germinal disc, which is not separated by a sharp line from the remainder of the ovum. Yolk spherules are present in this disc as elsewhere, but are much smaller and of a different character. The segmentation has the normal meroblastic character (fig. 15) and is confined to the germinal disc. Before it commences the germinal disc exhibits amoeboid movements. During the segmentation nuclei make their appearance spontaneously (?) in the yolk adjoining the germinal disc (fig. 15, _nx´_), and around them portions of the yolk with its protoplasmic network become segmented off. Cells are thus formed which are added to those resulting from the segmentation proper. Even after the segmentation numerous nuclei are present in the granular matter below the blastoderm (fig. 16 A, _n´_); and around these cells are being continually formed, which enter the blastoderm, and are more especially destined to give rise to the hypoblast. The special destination of many of these cells is spoken of in detail below. [18] _Vide_ Vol. II., p. 62. [FIG. 15. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO DURING THE SEGMENTATION. _n._ nucleus; _nx._ nucleus modified prior to division; _nx´._ modified nucleus in the yolk; _f._ furrow appearing in the yolk adjacent to the germinal disc.] At the close of segmentation the blastoderm forms a somewhat lens-shaped disc, thicker at one end than at the other; the thicker end being the embryonic end. It is divided into two strata--an upper one, the epiblast--formed of a single row of columnar cells; and a lower one, the primitive hypoblast, consisting of the remaining cells of the blastoderm, and forming a mass several strata deep. These cells will be spoken of as the lower layer cells, to distinguish them from the true hypoblast which is one of their products. [FIG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PRISTIURUS EMBRYO DURING STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE. _ep._ epiblast; _ll._ lower layer cells or primitive hypoblast; _m._ mesoblast; _hy._ hypoblast; _sc._ segmentation cavity; _es._ embryo swelling; _n´._ nuclei of yolk; _er._ embryonic rim. _c._ lower layer cells at the non-embryonic end of the blastoderm.]