REVERSAL OF CLEAVAGE IN ANCYLUS. 
SAMUEL J. HOLMES. 
Tue fact discovered by Crampton that in Physa, a genus of 
sinistral gasteropods, the direction of the early divisions of the 
ovum is just the reverse of the corresponding cleavages in the 
dextral forms suggests, as Crampton pointed out, that there may 
be some correlation between the reversed cleavage of the ovum 
and the reversed asymmetry of the adult. The reversed asym- 
metry of Physa is shown, not merely by the sinistral coil of the 
shell, but also by the arrangement of the organs of the body. 
In most gasteropods the anal and genital orifices open on the 
right side of the body, but in Physa their position is on the left 
side; and the lung cavity, normally on the right side in the 
pulmonates, opens on the left in Physa. In fact, the asym- 
metry of the sinistral form seems in all respects just the 
reverse of that of the dextral forms. In Planorbis, a genus 
in which there is a reversal of the position of the organs of the 
body, although the shell is often coiled in one plane or even 
dextral (pseudo-dextral), the cleavage of the ovum is of the 
same reversed type as in Physa. This reversal is shown as 
early as the second cleavage of the egg which leads to the 
four-cell stage. Even before there is any elongation of the 
dividing cell, or any external indication of cleavage whatever, 
the direction of the approaching division is shown by the incli- 
nation of the nuclear spindles. This early indication of the 
direction of division is shown in the two-cell stage in Crepidula, 
Limax, Lymnea, and Amphitrite, but in all these cases in which 
the cleavage is of the dexiotropic or normal type the inclination 
of the spindles is the reverse of that in Physa and Planorbis. 
As a consequence of this oblique division, we have probably in 
all forms with typical spiral cleavage a four-cell stage in which 
two of the cells lie at a somewhat lower level than the other 
two and meet in a cross furrow at the vegetal pole. The lower 
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