MATERIALS OF HEREDITY 1 3 



find out whether changing the chromosomes of the ferti- 

 lized egg changes the hereditary characters of the indi- 

 vidual. Suppose that, after the sperm and egg have united 

 in fertilization, one could reach in with a hook and pull 

 out one of the chromosomes; would that make any dif- 

 ference in the nature and characteristics of the individual 

 that develops from that egg? 



It has not been possible to perform this experiment in 

 just that way, but it has been possible to do what amounts 

 to the same thing. It has been possible to answer the ques- 

 tion; — What difference does it make to the new individual 

 if one of his chromosomes Is removed from the original 

 cell that produces him? The answer to that question will 

 be taken up in a moment. But before this is done, another 

 experiment must be proposed. 



It is of great interest to find out what difference it would 

 make if one of the chromosomes could be taken out. But 

 still more interesting would be the answer to the question: 

 What difference would it make if we could substitute one 

 chromosome for another? What difference would it make 

 if we could hook out one of the chromosomes, and put an- 

 other one, taken from another cell, in place of It? 



That question, too, it has been possible to answer. The 

 method of work has not been so crude as that of putting 

 in a hook and pulling out one chromosome, then poking 

 another Into Its place. But it has accomplished essentially 

 the same thing. 



Results of Removing a Chromosome, and of Substituting 

 One Chromosome for Another. — These two questions must 

 be kept in mind: (i) What difference does it make if one 

 of the chromosomes is removed from the original cell that 

 forms a new individual? And (2) what difference does it 

 make if we substitute one chromosome for another? 



The answers to these questions were found in what hap- 

 pens in nature Itself. It was found that In the ordinary 



