264 HANS PEZIBEAM 



i snail-shells, in individuals belonging to a species which is normally 

 asymmetrical in the opposite direction. To my knowledge there 

 exist no human families with situs inversus viscerum, and Lang 

 had no success in raising sinistral snails by inbreeding two sinistral 

 I individuals for two generations. We must, therefore, see in 

 I these instances of reversed asymmetry nothing else than somatic 

 transpositions, such as may also be induced artificially accord- 

 ing to Crampton, by compressing the snail-eggs. And as was 

 \ already suggested by Conklin the transpositions are brought about 

 'by the inversion of the relative position of the dorso-ventral and 

 antero-posterior anlagen. But in the germ-cells of the inverted 

 bod}'' the normal distribution reappears, thus indicating the 

 normal dextrosity of the dorso-ventral and antero-posterior 

 anlage,^" 

 " Closely analogous to this seem to be the minor asymmetries 

 which have a fixed position from birth but later in life may be- 

 come inverted, as for instance in the case of the dextrochelous 

 crabs, whose fossil predecessors have already borne the big claw 

 attached to the right side of the body. 



Of the causes of these asymmetries and why in certain species 

 one side of the body is particularly predisposed to modification, 

 we know nothing, but it was not even my purpose to discuss 

 here this matter. I merely wished to point out that the study of 

 minor asymmetries yields to us the key to the problem of bilater- 

 ality, with which we may now attempt to unlock the doors 

 separating us from the complete understanding of this problem. 



