The Cellular Basis 



195 



FIG. 72. NORMAL AND INVERSE SYMMETRY IN LATE EMBRYOS AND ADULT 

 STAGES. In i, cross-hatched area is blastopore ; cells shaded by lines, meso- 

 derm ; other cells, endoderm ; the spiral twist of the snail begins in oppo- 

 site directions in the two embryos. In 2, the adult organization is shown 

 with all organs inversely symmetrical; os, olfactory organ, a, anus; L, 

 lung; V, ventricle; K, kidney. In 3, sinistral and dextral shells of adult 

 snails are shown. 



