EFFECTS ON EGGS OF FRESH WATER PULMONATES 443 
and especially the spindle is displaced with difficulty, by centri- 
fugal force, it seems probable from many observations that its posi- 
tion in the cell is determined by something within the cell body. 
This has been shown to be the case in Crepidula (Conklin, 1902), 
where the future position and direction of the spindles are, in some 
cases, proclaimed by the form of the cytoplasm before the spindles 
are formed. 
In 1903 I suggested that the inversion of the symmetry of sin- 
istral, as compared with dextral snails, might be due to an inver- 
sion of the polarity of the egg at the time of maturation. I had 
hoped that by centrifuging the eggs of dextral and of sinistral 
snails at very early stages I might be able to artificially invert 
their symmetry. However, this hope has not been realized except 
in a few doubtful cases. In a few instances there developed from 
centrifuged eggs small snails with wide-mouthed conical shells 
which usually showed no spiral twist; and in one case (Exp. 18) 
two small snails developed, but did not hatch, in which the apex 
of the cone-shaped shell was turned forward and to the right, 
whereas in the normal snails of this species (Physa ancellaria) 
the apex of the shell turns backward and to the left. In one of 
these two snails the heart, which was easily recognized by its 
beating, was on the mid-dorsal line, in the other (fig. 46) it was 
on the left side, whereas in normal specimens it is on the right. 
This represented a partial inversion of sj^mmetry, at least, but 
since such inversion occurred only twice among hundreds of centri- 
fuged eggs, and since it never occurred in isolated eggs, the history 
of which was known from the beginning, it cannot be affirmed 
that it was produced by an inversion of polarity at the time of 
centrifuging, and its precise cause remains unknown. 
In the paper referred to I mentioned the fact that the polar 
bodies in Crepidula might be caused to be extruded at the vege- 
tative pole, and I assumed that the polarity of the egg might thus 
be inverted. I did not then know that the final polarity of this 
egg might be unchanged, irrespective of the position which the 
polar spindles and the polar bodies may be forced to assume. In 
view of the fact that it is quite impossible by the methods hitherto 
employed to invert the polarity of the egg, it is evident that the 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 9, NO. 2. 
