184 HEREDITY AND ENVIRONMENT 



whereas those of the egg persist. In short 

 ontogeny begins in the egg before fertiliza- 

 tion whereas the sperm can influence ontogeny 

 only after, and usually a considerable time 

 after, it enters the egg. 



The fact remains that at the time of fertiliza- 

 tion the hereditary potencies of the two germ 

 cells are not equal, 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 determines the early 

 development and the sperm and egg nuclei 

 control only later differentiations. 



We are vertebrates because our mothers 

 were vertebrates and produced eggs of the 

 vertebrate pattern; but the color of our skin 

 and hair and eyes, our sex, stature, and mental 

 peculiarities were determined by the sperm as 

 well as by the egg from which we came. There 

 is evidence that the chromosomes of the egg 

 and sperm are the seat of the differential fac- 

 tors or determiners for Mendelian characters, 



