THE CELLULAR BASIS 171 



stages it is certain that the bilateral symmetry 

 of the egg becomes the bilateral symmetry of 

 the developed animal. 



(c) Inverse Symmetry. — In most animals 

 bilateral symmetry is not perfect, certain or- 

 gans being foimd on one side of the mid line 

 and not on the other, or being larger or dif- 

 ferently located on one side as compared with 

 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 niay 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 IcnoAvn 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, 

 41 ) . There is good reason to believe that in 



