72 The Third and Fourth Generation 



one weighty reason for believing that it is the 

 physical basis of hereditary transmission; other 

 reasons follow. The sperm consists of a head 

 made almost exclusively of chromatin, and of 

 a middle piece, and tail that propels the sperm 

 to the egg. These parts are often dropped at 

 time of fertilization, so that only the head unites 

 with the egg. Yet the plant or animal resulting 

 from the fertilized egg may show quite as many 

 hereditary traits derived from the father's 

 ancestry as from the mother's; these evidently 

 must have been brought into the egg by the 

 chromatin. Boveri was able to shake the 

 nucleus out of certain large sea-urchin eggs and 

 then fertilize these denucleated eggs with the 

 sperm of another species. When these eggs 

 developed they showed the specific characters 

 of the latter only; none of the characteristics 

 of the species from which the eggs were derived 

 were apparent. He also found that under 

 exceptional conditions, when sea-urchin eggs 

 were fertilized with sperm of the same species, 

 two or even more sperms would enter the same 

 egg. Then the chromosomes are distributed 



