The Cellular Basis 199 



either by natural or by artificial parthenogenesis, and in such cases 

 the characteristic polarity, symmetry and pattern of the adult are 

 found in the cytoplasm of the egg just as if the latter had been 

 fertilized. The conclusion seems to be justified that these earliest 

 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 

 egg and of the developed animal are found in the cytoplasm. No 

 doubt the differentiations of the cytoplasm of the egg as well as 

 the peculiar form and structure of the spermatozoon have arisen, 

 during the genesis of these cells, under the influence of paternal 

 and maternal chromosomes as well as of the environment, just 

 as in the differentiation of any tissue cell; but in the case of the 

 spermatozoon these cytoplasmic differentiations are lost when it 

 enters the egg, whereas those of the egg persist. In short on- 

 togeny begins in the egg before fertilization 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 fertilisation the poten- 

 cies of the two germ cells are not equal, the polarity, sym- 

 metry, type of cleavage, and the pattern, or relative positions and 

 proportions of future organs, being foreshadowed in the cyto- 

 plasm of the egg cell, while only the differentiations of later de- 

 velopment are influenced by the sperm. In short the egg cyto- 

 plasm 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. The chromosomes of the egg and sperm are the seat of 

 the differential factors or determiners for Mendelian characters, 



