MATURATION AND IMPREGNATION OF THE OVUM. 531 



^sjg^V^V-olf'*?^^"^^^^^ 



FIG. 5. Ovum of Asterias glacialis, at the same stage as Fig. 4, treated with picric 



acid (copied from Fol). 



Fol concludes that the spindle is formed out of part of the 

 germinal vesicle and not of the germinal spot, while he sees in 

 the round body present in the lower of the two clear spaces the 

 metamorphosed germinal spot. He will not, however, assert 

 that no fragment of the germinal spot enters into the formation 

 of the spindle. It may be observed that Fol is here obliged to 

 fill up (so far at least as his present preliminary account enables 

 me to determine) a lacuna in his observations in a hypothetical 

 manner, and O. Hertwig's (13) most recent observations on the 

 ovum of the same or an allied species of Asterias tend to throw 

 some doubt upon Fol's interpretations. 



The following is Hertwig's account of the changes in the 

 germinal vesicle. A quarter of an hour after the egg is laid the 

 protoplasm on the side of the germinal vesicle towards the 

 surface of the egg develops a prominence which presses inwards 

 the wall of the vesicle. At the same time the germinal spot 

 develops a large vacuole, in the interior of which is a body con- 

 sisting of nuclear substance, and formed of a firmer and more 

 refractive material than the remainder of the germinal spot. In 

 the above-mentioned prominence towards the germinal vesicle, 

 first one sun is formed by radial striae of protoplasm, and then a 

 second makes its appearance, while in the living ovum the 

 germinal spot appears to have vanished, the outline of the 

 germinal vesicle to have become indistinct, and its contents to 

 have mingled with the surrounding protoplasm. Treatment 

 with reagents demonstrates that in the process of disappearance 

 of the germinal spot the nuclear mass in the vacuole forms a 



