THE MECHANISM OF REPRODUCTION 49 



and for this reason will transmit color-blindness to one- 

 half of their sons by a normal man, as will be seen by 

 following out the fourth and fifth columns in the diagram. 

 An egg containing the normal x-element can meet a sper- 

 matozoon carrying an x-element and thus produce a 

 daughter, or it may meet a spermatozoon with no x-ele- 

 ment and thus produce a son ; but in either case the chil- 

 dren will have normal vision. On the other hand, an egg 

 containing a defective x-element will by similar fertiliza- 

 tions result either in a normal-visioned daughter, who will 

 carry color-blindness in half of her ova, or in a son who 

 will be color-blind. 



Such a scheme of interpretation might seem quite 

 visionary were it not for the fact that similar types of 

 inheritance occur in many of the lower animals. By care- 

 fully controlled experiments with them it has been proven 

 beyond a doubt. 



