Origin of Different Kinds of Adaptations 345 
found an individual with the spiral in the opposite direction 
(sinistral), and in this the conditions of the internal organs 
are exactly reversed. It is the left set of organs that is now 
atrophied, and the right set that is functional. Such changes 
appear suddenly. Organs of one side of the body that have 
not been functional for many generations may become fully 
developed. Moreover, Lang has shown that when a sinis- 
tral form breeds with a normal dextral form, or even when 
sinistral forms are bred with each other, the young are prac- 
tically all of the ordinary type. 
An attempt has been made to connect these facts with the 
mode of development of the mollusks. It is known that the 
eggs of a number of gasteropod mollusks segment in a per- 
fectly definite manner. A sort of spiral cleavage is followed 
by the formation of a large mesodermal cell from the left 
posterior yolk-cell. From this mesodermal cell nearly all 
the mesodermal organs of the body are formed. Thus it 
may appear that the spiral form of the snail is connected 
with the spiral form of the cleavage. In a few species of 
marine and, fresh-water snails the cleavage spiral is reversed, 
and the mesoderm arises from the right posterior yolk-cell. 
It has been shown in several cases that the snail coming 
from such an egg is twisted in the reverse direction from 
that of ordinary snails. 
It has been suggested, therefore, that the occasional sinis- 
tral individual of Helix arises from an egg cleaving in the 
reverse direction, and there is nothing improbable in an 
assumption of this kind. No attempt has been made as yet 
to explain why, in some cases, the cleavage spiral is turned 
in one direction, and in other cases in the reverse direction ; 
but even leaving this unaccounted for, the assumption of 
the unusual form of Helix being the result of a reversal of the 
cleavage throws some light as to how it is possible for the 
complete reversal of the organs of the adult to arise. If it is 
assumed that in the early embryo the cells on each side of 
