74 Heredity. 



esis," we certainly should look for the occurrence of 

 parthenogenesis in ripe ova which have not extruded 

 these bodies. 



However this may be, the correctness or incorrect- 

 ness of the polar-cell hypothesis has no bearing upon 

 our present argument, for the phenomena of partheno- 

 genesis show beyond question that an egg may develop 

 without union with a male cell, and there is no evidence 

 whatever that a male cell ever acts in a similar way. 



Other reasons for believing that the ovum and the male 

 cell perform different functions in heredity. 



Even if the possibility of parthenogenesis did not 

 show us that the part played in heredity by the ovum is 

 different from that played by the male cell, there are 

 many other reasons for believing that the difference in 

 the form of the two sexual elements corresponds to some 

 profound difference of function. 



I shall devote several chapters of this book to the ex- 

 tended discussion and proof of the facts which drive us 

 to this conclusion, and I shall show that the belief in 

 the essential similarity of the functions of the repro- 

 ductive elements cannot possibly be retained. 



When the male of one species or variety is crossed 

 with the female of another species or variety, the hybrid 

 offspring is often very different from that which is 

 produced when the female of the first species is crossed 

 with the male of the second. If the function of the 

 ovum is the same as that of the male cell, we should 

 have exactly the same elements in each case, and should 

 expect the same result. The fact that the result is not 

 the same proves that the elements are not the same 

 either. 



In many cases the male of one species will breed 



