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-i 

 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 



