The nature of the changes which take place in the maturation of the ovum may perhaps be most conveniently displayed by following the history of a single ovum. For this purpose the eggs of Asterias glacialis, which have recently formed the subject of a series of beautiful researches by Fol (87), may be selected. The ripe ovum (fig. 22), when detached from the ovary is formed of a granular vitellus enveloped in a mucilaginous coat, the zona radiata. It contains an eccentrically-situated germinal vesicle and a germinal spot. In the former is present the usual protoplasmic reticulum. As soon as the ovum reaches the sea-water the germinal vesicle commences to undergo a peculiar metamorphosis. It exhibits frequent changes of form, the reticulum vanishes, its membrane becomes gradually absorbed, its outline indented and indistinct, and finally its contents become to a certain extent confounded with the vitellus (fig. 23). [FIG. 23. TWO SUCCESSIVE STAGES IN THE GRADUAL METAMORPHOSIS OF THE GERMINAL VESICLE AND SPOT OF THE OVUM OF ASTERIAS GLACIALIS IMMEDIATELY AFTER IT IS LAID (copied from Fol).] The germinal spot at the same time loses its clearness of outline and gradually disappears from view. At this stage, and between it and the stage represented in fig. 26, the action of reagents brings to light certain appearances the nature of which is not yet fully cleared up for Asterias, which have been described somewhat differently by Fol for Ast. glacialis and Hertwig for Asteracanthion. Fol finds immediately after the stage just described that a star is visible between the remains of the germinal vesicle and the surface of the egg, which is connected with an imperfectly-formed nuclear spindle extending towards the germinal vesicle[28]. At the end of the nuclear spindle may be seen the broken up fragments of the germinal spot. [28] By the term 'nuclear spindle' I refer to the peculiar form of a double striated cone assumed by the nucleus just before division, which is no doubt familiar to all my readers. I use the term star for the peculiar stellate figure usually visible at the poles of the nuclear spindle. For a further description of these parts the reader is referred to Chapter IV. [FIG. 24. OVUM OF ASTERIAS GLACIALIS SHEWING THE CLEAR SPACES IN THE PLACE OF THE GERMINAL VESICLE. FRESH PREPARATION (copied from Fol).]