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condition. Such • cases of inverse symmetry have been observed in 

 many other vertebrates as well as in certain invertebrates. Among 

 the latter the best known cases are those presented by sinistral 

 gasteropods in which not only is the shell wound in a left spiral 

 instead of a right one but all the organs are apparently transposed 

 with respect to the plane of symmetry; thus the single kidney, gill 

 and osphradium are found on the right side instead of the left and 

 the nervous, digestive, and circulatory systems show a complete in- 

 version of the usual form of asymmetry. Zur Strassen (1896) has 

 described a similar reversal of asymmetry in Ascaris and it is probable 

 that such reversals occur in many other groups of animals though 

 they can of course be observed only in asymmetrical parts. Organs 

 which are perfectly bi-symmetrical might undergo complete inversion 

 without its ever being apparent. The cause of such inversion of 

 symmetry has heretofore remained unknown, though it is a most 

 interesting and important question and one of wide and fundamental 

 significance. 



In many respects the gasteropods offer the most favorable opportun- 

 ity for the study of this problem for here are found closely related 

 species and genera presenting totally reversed forms of asymmetry, as 

 for example Succinea and Limnaea which are typically dextral and 

 Physa and Planorbis which are typically sinistral; moreover dextral 

 and sinistral forms may occur even within the same species, as for 

 example in the garden snail of Europe, Helix pomatia, in different 

 species of Campeloma, in Ascaris megalocephala etc. It is certainly 

 a most interesting and remarkable fact that such apparently profound 

 modifications of structure may be found within the limits of a single 

 species. Since the phylogeny must be the same for all individuals of 

 a species it follows that such individual differences must arise in the 

 course of the ontogeny and it has, therefore, seemed to me not im- 

 possible that the cause of this remarkable condition might be de- 

 termined by a sufficiently careful study of the individual development. 



Crampton (1894) and Kofoid (1894) first pointed out the im- 

 portant fact that in the sinistral snails, Physa and Planorbis, the 

 cleavage of the egg from the second division onward is typically spiral 

 like that of dextral snails, except that it is totally reversed in direction. 

 Zur Strassen (189(5) found that in Ascaris megalocephala about one 

 egg out of thirty to forty segmented in an inverse manner and 

 correspondingly he found that out of 125 adults four showed inverse 

 symmetry; he concluded therefore that the inverse symmetry of the 

 adult was causally related to the inverse cleavage of the egg, "oder 



