THE CELLULAR BASIS 177 



the other; among all such animals variations 

 occasionally occur which show a complete re- 

 versal of these asymmetrical organs, i. e., in 

 man the heart and arch of the aorta may occur 

 on the right side instead of the left, the pyloris 

 and chief portion of the liver on the left in- 

 stead of the right, etc. Among certain snails 

 this inversion of symmetry may occur regu- 

 larly in certain species and not in others, the 

 inverse form being known as sinistral and the 

 ordinary form as dextral (Fig. 42). In these 

 sinistral snails, and probably in all animals 

 showing inverse symmetry, the embryo is in- 

 versely symmetrical and every cleavage of the 

 egg from the first to the last is the inverse of 

 that which occurs in dextral snails (Figs. 40- 

 42 ) . There is good reason to believe that in 

 such cases the unsegmented egg is also in- 

 versely symmetrical as compared with the 

 more usual type (Fig. 40). In all of these 

 cases there is a direct correspondence between 

 the polarity and symmetry of the oosperm 

 and the polarity and symmetry of the devel- 

 oped animal (Figs. 38-42). 



(d) Localization Pattern. — In many ani- 



