Fig. 8.—Portion of the ovum of Asterias glacialis, with the first polar body as it appears when living (copied from Fol).
Fig. 9.—Portion of the ovum of Asterias glacialis immediately after the formation of the second polar body. Picric acid preparation (copied from Fol).
The spindle, however it be formed, has up to this time been situated with its axis parallel to the surface of the egg, but not long after the stage last described a spindle is found with one end projecting into a protoplasmic prominence which makes its appearance on the surface of the egg (Fig. 6). Hertwig believes that the spindle simply travels towards the surface, and while doing so changes the direction of its axis. Fol finds, however, that this is not the case, but that between the two conditions of the spindle an intermediate one is found in which a spindle can no longer be seen in the egg, but its place is taken by a compact rounded body. He has not been able to arrive at a conclusion as to what meaning is to be attached to this occurrence. In any case the spindle which projects into the prominence on the surface of the egg divides it into two parts, one in the prominence and one in the egg (Fig. 7). The prominence itself with the enclosed portion of the spindle becomes partially constricted off from the egg as the first polar body (Fig. 8). The part of the spindle which remains in the egg becomes directly converted into a second spindle by the elongation of its fibres without passing through a typical nuclear condition. A second polar cell next becomes formed in the same manner as the first (Fig. 9), and the portion of the spindle remaining in the egg becomes converted into two or three clear vesicles (Fig. 10) which soon unite to form a single nucleus, the female pronucleus (Fig. 11). The two polar cells appear to be situated between two membranes, the outer of which is very delicate and only distinct where it covers the polar cells, while the inner one is thicker and becomes, after impregnation, more distinct and then forms what Fol speaks of as the vitelline membrane. It is clear, as Hertwig has pointed out, that the polar bodies originate by a regular cell division and have the value of cells.
Fig. 10.—Portion of the ovum of Asterias glacialis after the formation of the second polar cell, shewing the part of the spindle remaining in the ovum becoming converted into two clear vesicles. Picric acid preparation (copied from Fol).
Fig. 11.—Ovum of Asterias glacialis with the two polar bodies and the female pronucleus surrounded by radial striæ, as seen in the living egg (copied from Fol).