176 HEREDITY AND ENVIRONMENT 



may be carried over from generation to gen- 

 eration through the egg cell, but the symmetry 

 and localization pattern develop in the ovum 

 before or just after maturation. In this dif- 

 ferentiation and localization of the egg cyto- 

 plasm it is probable that certain influences 

 have come from the nucleus of the egg, and 

 perhaps from the egg chromosomes. There is 

 no doubt that most of the differentiations of 

 the egg cytoplasm have arisen during the 

 ovarian history of the egg, and as a result of 

 the interaction of nucleus and cytoplasm; but 

 the fact remains that at the time of fertiliza- 

 tion the hereditary potencies of the two germ 

 cells are not equal, all the early stages of de- 

 velopment, including the polarity, symmetry, 

 type of cleavage, and the pattern, or relative 

 positions and proportions of future organs, 

 being foreshadowed in the cytoplasm of the 

 egg cell, while only the differentiations of later 

 development are influenced by the sperm. In 

 short the egg cytoplasm fixes the general type 

 of development and the sperm and egg nuclei 

 supply only the details. 



We are vertebrates because our mothers 



