154 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Tlie nuclear membrane is very distinct up to the time when the ovum approaches 
maturity (figs. 159 and 160), when its outlines have become hazy. In the case illus- 
trated (fig. 140) the long diameter of the nucleus corresponds with the short diameter 
of the egg. The nucleolus and nuclear fluid have undergone a very marked change. 
When stained in Kleinenberg’s lnemotoxylon, the nucleolus has a hazy, almost 
homogeneous, appearance, and stains rather feebly, while the karyoplasm is of the 
same character, but takes the stain more feebly still. 
When the eggs are ripe and lie free within the ovary ready for extrusion (fig. 141, 
plate 39), it is difficult to find the nucleus (partly, no doubt, on account of the great 
mass of yolk and the difficulty in cutting it). In one case, where I succeeded, what 
appeared to be the metamorphosed nucleus was a somewhat eccentric island of karyo- 
plasm (fig. 160; for position in ovum, see fig. 141) without membrane or trace of a 
nucleolus. This vesicle stains uniformly, and has a very fine-grained texture. It 
has started for the surface of the egg, and in the next stage examined (fig. 161) is in 
contact with it. In this particular egg, taken from the oviduct of a female shortly 
after ovulation, the cell is dividing, or giving off a polar body; the nucleus has dimin- 
ished in size, and no membrane is distinguishable. 
THE MOVEMENTS OF THE NUCLEOLUS THROUGH THE ACTION OF GRAVITY. 
I have already pointed out the eccentric position of the nucleolus, which is always 
observed whenever the immature ovary is sectioned. This was noticed by Bnmpus in 
1891, but no explanation of the fact was offered. {30, p. 225.) 
Cut 18. — From transverse section of a part of ovary of lobster, hardened with ventral side uppermost, to show the 
effect of gravity upon the nucleolus. From hard-shell lobster which had recently hatched a brood. July 18, 1894. 
Cut 19. — From transverse section of a part of same ovary, hardened with dorsal side uppermost, to show the effect of 
gravity upon the nucleolus. _D, dorsal surface of ovary; nucleus of ovum; ncl , nucleolus of ovum; ow, ovarian wall; 
V } ventral surface of ovary. 
The arrow in each cut shows the direction of the force of gravity. 
It seemed very probable that this phenomenon was due to gravity acting directly 
upon the nucleolus, which was free to move in every part of the nucleus. A few simple 
experiments immediately proved that this was the case. The ovary of a lobster which 
had recently hatched a brood was selected and cut into several pieces. These were 
then hardened in different positions, in Mayer’s picro-sulphuric acid, with ventral or 
D 
Cot 18. 
Cut 19. 
