58 HEREDITY AND SEX 



cells contain a complete assortment. If normal devel- 

 opment depends on an embryo containing in every cell 

 at least one of each kind of chromosome, then in 

 our simple case only one group of four cells has this 

 possibility. 



Boveri found that such dispermic eggs produce 

 normal embryos very rarely. He calculated what the 

 chance would be when three times 18 chromosomes 

 are involved. The chance for normal development 

 is probably not once in 10,000 times. He isolated 

 many dispermic eggs and found that only one in 1,500 

 of the tetrad type developed normally. 



Boveri went still further in his analysis of the prob- 

 lem. It had been shown for normal eggs that if at 

 the two-celled stage the cells are separated, each forms 

 a perfect embryo. This is also true for each of the 

 first four cells of the normal egg. 



Boveri separated the four cells of dispermic eggs and 

 found that the quadrants not infrequently developed 

 normally. This is what we should anticipate if those 

 cells can develop that contain one of each kind of 

 chromosome. 



The evidence furnishes strong support of the view 

 that the chromosomes are different from each other, 

 and that one of each kind is necessary if development 

 is to take place normally. 



The evidence that Baltzer has brought forward is 

 also derived from a study of sea-urchin eggs. It is 

 possible to fertilize the eggs of one species with sperm 

 of another species. The hybridizing is greatly helped 

 by the addition of a little alkali to the sea water. 



Baltzer made combinations between four species of 



