THE CELLULAR BASIS OF HEREDITY 239 



dextral snails (Figs. 42, 43). There is good reason to believe that in 

 such cases the unsegmented egg is also inversely symmetrical as com- 

 pared with the more usual type (Kg. 42). In all of these cases there 

 is a direct correspondence between the polarity and symmetry of the 



Fig 42. Inverse Symmetry in the Unsegmented Egg and in the First and 

 Second Cleavages. Figs. 42, 43, 44. The cause of inverse symmetry in snails. In 

 each case the right-hand column represents dextral forms, the left-hand column sinis- 

 tral ones. 



oosperm and the polarity and symmetry of the developed animal (Fig. 

 40-44). 



(d) Localization Pattern 



In many animals the ectoderm, endoderm and mesoderm may be 

 traced back to areas of peculiar protoplasm in the oosperm, but in 

 addition to this one can recognize in the ascidian egg areas of peculiar 

 protoplasm which will give rise to mesenchyme, muscles, nervous system 

 and notochord, and these substances are present in the oosperm in the 



