222 The Significance of Sex. [March 
the plane of cleavage. It was in 1862 that Robin gave them the 
name they now usually bear. 
In 1842, Bischoff saw the germinal vesicle expelled from the 
egg during maturation, and this was confirmed by other observers, 
and thus the idea that the polar globules were the extruded ger- 
minal vesicle was gradually established. 
In 1853, Keber discovered the micropyle, and the theory o 
actual penetration of spermatozoa into the egg thus received 
more favor, speculations concerning the functions of the sperma- 
tozoon became more numerous. Bischoff held the katalytic 
theory, by which molecular motion was supposed imparted to 
the egg through the spermatozoon. Meissner thought it was a 
nutriment, others thought it served to help maturation, and thus 
for a long time the formation of polar globules was supposed 
to depend on fertilization. The independence of these phenoment 
was shown in 1875 by Hertwig. 
The penetration of more than one spermatozoon was seen by 
several observers, and it was only gradually that the idea gained 
ground that normally but one spermatozoon enters the egg. 
Perez thought, in 1879, that there may be degrees of partheno- 
genesis, so that if this is strong in tendency, it does not take as 
many spermatozoa to saturate the ovum as if weak. 
The next step was the discovery of the sexual pronuclei. The 
male pronucleus (so termed by Fol) was first seen by Weil in 
1873, but its direct morphological connection with the head of a 
spermatozoon was first established by Hertwig in 1875. Hert- 
wig also showed that the whole germinal vesicle was not ex- 
truded in the polar globules, but that the germinal dot remained 
to be transformed into the female pronucleus, which fused with 
the male pronucleus. Auerbach had seen these pronuclei fuse, 
but supposed they originated in opposite poles of the egg, and by 
uniting, the characters of the different hemispheres of the egg 
would be mixed. Beneden and Bütschli practically saw the 
same thing bie, but likewise derived these bodies by endoge- 
nous formation. Fol was, however, successful in seeing the 
female pronucleus derived from the amphiaster which extruded 
the polar globule ; but it remained for Hertwig, in 1877, to show 
that the polar bo dies arise 
, an sie ian tlie kneiadegie Of the fomai pror 
i and Giard arrived at this result independently 
. 
arise by a true karyokinetic division of the 
