SEXUAL AND ASEXUAL REPRODUCTION 259 



does unite with the egg nucleus, abnormal results arise and 

 no proper embryo develops. In the vast majority of cases, 

 however, the single sperm unites with the single egg nucleus, 

 and all other sperms that chance to be present have nothing 

 to do with the development, but soon disappear. 



THE RELATION OF THE CHROMATIN TO HEREDITY 



The facts just mentioned show us that the chromatin must 

 play a very important part in the transmission of characters 

 from parent to offspring. It is a demonstrated fact that 

 both the male and the female parents can transmit their 

 characters equally to their offspring. It follows that both 

 parents would probably transmit an equal amount of heredi- 

 tary substance to the next generation. The process of fertili- 

 zation just described shows that the only parts contributed 

 by the male parent to the fertilized egg are the centrosome 

 and the chromosomes. Hence whatever the male parent trans- 

 mits to its offspring must be contained either in the centrosome 

 or the chromosomes. But the female parent does not contribute 

 any centrosome to the combined egg, and it should be remem- 

 bered that in plants there is no centrosome. The female does 

 contribute an amount of chromatin equal to that which the 

 male contributes, namely, in the case described, two chromo- 

 somes. This fact proves that the chromosomes must certainly 

 contain hereditary material. These chromosomes are extremely 

 minute, far below the reach of the human vision and only 

 seen with a high-power microscope and by special microscopic 

 methods. It seems almost incredible that there can be in 

 such a small compass the traits of characters which an indi- 

 vidual transmits to its offspring and which the offspring in- 

 herits from its parents. But the facts described seem to be 

 capable of no other interpretation, and we are therefore justi- 

 fied in saying that the chromatin material is the bearer of 

 heredity. This does not necessarily mean that other parts 

 jf the egg and sperm may not have some share in heredity. 



