154 



BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The 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) tlie 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 haemotoxylon, 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 And 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 (tig. 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 Bumpus in 

 1891, but no explanation of the fact was offered. (30, p. 225.) 



D 



Cut 19. 



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 ; nc, nucleus of ovum ; nel, 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 |)Ositions, in Mayer's picro-sulphuric acid, with ventral or 



