KARYOKINESIS AND ITS RELATION TO FERTILIZATION, 227 
egg-protoplasm, and after fertilization collects again to form a new nucleus. 
Auerbach (as already mentioned) believed that the nuclear substance was 
really dissolved. EE. van Beneden came to the conclusion in his work on the 
formation of eggs (18), (see especially pp. 239, et seq.) that the invisibility of 
the germinal vesicle is no proof that it really disappears, or that it is dis- 
solved in the yolk. It is probably a change in the yolk that has caused this 
invisibility ; but he thinks it is just as likely that there is a change in the 
germinal vesicle itself which brings about this invisibility. The sequel will 
show how far this is true. A. Brandt (87) believes neither in subdivision, 
nor solution, nor extrusion,—he traces the apparent disappearance of the 
germinal vesicle rather to an active ameboid movement, which is performed 
by the germinal vesicle and spot, before impregnation. The same view is 
held by Schneider (181). But the recent researches of Bitschli, of Fol, 
Flemming, Hertwig, and lastly and especially, of E. van Beneden and Oskar 
Schultze, show that in the formation of the directive corpuscles a portion 
of the germinal vesicle is extruded, in order to form the directive corpuscles, 
and that in this process such a change is brought about in the germinal vesicle 
that its apparent disappearance can readily be explained. O. Hertwig origin- 
ally thought that the greater part of the germinal vesicle disappeared, and 
that the germinal spot remained as the new egg-nucleus; but later on he 
changed his opinion in the way above mentioned. Biitschli was the first to 
prove the connection between the disappearance of the germinal vesicle and 
the appearance of the directive corpuscles, as well as the formation of the 
spindle figure in the origin of the latter, so that the now generally received 
view that the extrusion of the directive corpuscles is to be regarded asa 
karyokinetic division of the egg-cell with very unequal products, obtained its 
first sound basis. Carnoy has supported this opinion by his description of 
the formation of a cell-plate during the process. Cf. on the history of the 
directive corpuscles, the memoirs of Platner and Robin. 
The most detailed figures of the whole process are given by 
O. Hertwig (1. c.) and Fol (1. c.) for Worms and Echinoderms ; 
by Schneider (I. c.), Nussbaum (l. c.), E. van Beneden (I. c.), 
for Ascaris megalocephala; by Carnoy (I. c.), Zacharias 
(l. c.), and recently by Boveri (1. c.) for the same animal. 
For Vertebrates Oskar Schultze (182) has published a detailed 
research on the process in Amphibia. 
I will briefly recapitulate the results of the latter, in order 
to render the process more intelligible by an example. 
In Amphibia two directive corpuscles are extruded, in Rana 
one before, the other after fertilization (cf. Kupfer’s state- 
ment for Petromyzon). In Rana the two corpuscles are 
