874 THE AMERICAN NATURALIST.  [VOL. XXXIII. 
the egg become so arranged as to produce the appearance of 
a cross, the center of which lies at the apical pole. It was 
found by Conklin that the arms of the cross in Crepidula are 
not exactly meridional in position, but have a slight dexiotropic 
twist. This twist Conklin considered as an expression of the 
dexiotropic cleavage of the egg, and the fact that the same 
inclination of the arms of the cross has been found by Heath 
in Ischnochiton, a form with the same type of cleavage, tends 
to confirm this view. In Planorbis, on the other hand, the 
inclination of the arms of the cross is very plainly laeotropic, 
and this twist may be seen as late as the end of the period of 
gastrulation. A careful comparison of the genesis of the arms 
of the cross in these forms would convince any one, I believe, 
that the different directions of the inclination of the arms is 
correlated with the different types of cleavage. As a result, 
therefore, of reversed cleavage a considerable portion of the 
embryo becomes subjected to a torsion in a direction different 
from that which occurs in the normal forms. It does not seem 
possible in Planorbis to connect directly this torsion of the 
cross with the sinistral organization of the animal, for this 
structure disappears before the asymmetry of the embryo is 
manifest. However, the fact that reversal of cleavage shows 
its effects as late as the gastrula stage lends considerable sup- 
port to the view that it may stand in some causal relation to 
the reversed asymmetry of the adult. 
In Crepidula, Conklin has succeeded in tracing the beginning 
of the asymmetry of the embryo to the division of one of the 
cells of the entoderm. The time and direction of this cleavage 
were found to give the initial twist to the rudiment of the intes- 
tine. “If,” says Conklin, “the initial asymmetry is caused in 
Physa, as in Crepidula, by the asymmetry of the cells 5C and 
5D, then it is easy to see how this reversal of cleavage stands 
in a causal relation to the reversed asymmetry of the adult.” 
In Planorbis the corresponding divisions result in the forma- 
tion of very small cells instead of large, yolk-laden ones, as in 
Crepidula, and do not seem to exercise any influence in deter- 
mining the asymmetry of the embryo. The cells of the ento- 
derm become very numerous (over thirty) and of small size 
