194 



Heredity and Environment 



FIG.. 71. NORMAL AND INVERSE SYMMETRY OF THE 30, 4TH, STH AND 6'm 

 CLEAVAGES. The cells la-id, 2a-2d and 3a-3d give rise to all the ectoderm ; 

 4d or M gives rise to mesoderm ; A, B, C, D to endoderm. 



and most fundamental differentiations which distinguished the 

 eggs of various phyla are not dependent upon the entering sperma- 

 tozoon. 



Share of Egg and Sperm in Heredity. All of these corre- 

 spondences between the polarity, symmetry and pattern of the 



