Giard (10) gives a somewhat different account of the behaviour of the germinal vesicle in Psammechinus miliaris. At maturity the contents of the germinal vesicle and spot mix together and form an amœboid mass, which, assuming a spindle form, divides into two parts, one of which travels towards the centre of the egg and forms the female pronucleus, the other remains at the surface and gives origin to two polar cells, both of which are formed after the egg is laid. What Giard regards as the female pronucleus is perhaps the lower of the two bodies which take the place of the original germinal vesicle as described by Fol. Vide the account of Fol's observations on p. [531].
Strasburger, from observations on Phallusia, accepts in the main Hertwig's conclusion with reference to the formation of the polar bodies, but does not share Hertwig's view that either the polar bodies or female pronucleus are formed at the expense of the germinal spot alone. He has further shewn that the so-called canal-cell of conifers is formed in the same manner as the polar cells, and states his belief that an equivalent of the polar cells is widely distributed in the vegetable subkingdom.
This sketch of the results of recent researches will, it is hoped, suffice to bring into prominence the more important steps by which the problems of this department of embryology have been solved. The present aspects of the question 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 (9), may conveniently be selected.
The ripe ovum (Fig. 2), when detached from the ovary, is formed of a granular vitellus without a vitelline membrane, but enveloped in a mucilaginous coat. It contains an excentrically situated germinal vesicle and 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, its membrane becomes gradually absorbed and its outline indented and indistinct, and finally its contents become to a certain extent confounded with the vitellus (Fig. 3).
The germinal spot at the same time loses its clearness of outline and gradually disappears from view.
Fig. 2.—Ripe ovum of Asterias glacialis enveloped in a mucilaginous envelope, and containing an excentric germinal vesicle and germinal spot (copied from Fol).
Fig. 3.—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).