MATURATION AND IMPREGNATION OF THE OVUM. 529 



together and form an amoeboid 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 de- 

 scribed 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 un- 

 dergo a peculiar metamorphosis. It exhibits frequent changes 

 of form, its membrane becomes gradually absorbed and its out- 

 line 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. 



At a slightly later stage in the place of the original germinal 

 vesicle there may be observed in the fresh ovum two clear 

 spaces (fig. 4), one ovoid and nearer the surface, and the second 



