CELL SIZE AND NUCLEAR SIZE 65 
In fig. 11 an egg is shown which was centrifuged for ten minutes 
after the formation of the first polar body and before the formation 
of the second, the axis of centrifuging being such that the lighter 
protoplasm was thrown to the vegetative pole and the heavier 
yolk to the animal pole, thus reversing the normal positions of 
these substances. After centrifuging, the egg was left in sea water 
for three hours before being fixed. The first polar body, which 
has partially divided, lies at the animal pole; the second matura- 
tion spindle has been greatly elongated and its axis has been 
turned somewhat, its lower pole having been moved to the right 
in the figure. The egg has begun to constrict opposite the equa- 
tor of the spindle, thus leading to the formation of a giant sec- 
ond polar body. The nucleus of this second polar body consists 
only of a compact mass of chromosomes surrounded by yolk; 
the sphere connecting these chromosomes with the egg membrane 
is much elongated. The egg nucleus and sphere at the lower pole 
of the spindle are in contact with the field of cytoplasm and are 
much larger than those at the upper pole. The sperm nucleus 
and sphere, lying in the cytoplasmic field, are much the largest 
in the egg. In normal condition these relations are reversed, 
the sperm nucleus lying in the yolk, while the egg nucleus is in 
the cytoplasmic field; and in such cases the egg nucleus and sphere 
are larger than those of the sperm; however as the sperm nucleus 
approaches the egg nucleus and thus moves up into the cytoplasm 
it continually grows larger until, at the time the two meet, the 
sperm nucleus is almost as large as the egg nucleus. The fact that 
the normal size relations of these two nuclei may be reversed by 
reversing the positions of the cytoplasm and yolk, furnishes con- 
clusive evidence of the fact that the relative sizes of the egg and 
sperm nuclei and asters are dependent upon the quantity of cyto- 
plasm in which they lie. 
Furthermore fig. 11 shows that the spindle itself is a structure 
composed of fibers more firm than the surrounding substance, 
and is not merely an arrangement of the granules, which happen 
to be present in a field of force, into lines, like iron filings in a mag- 
netic field. The spindle remains fixed in position when all sur- 
rounding substances change position, and the spindle fibers, 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, No. 1 
