OPERATION OF THE GENETIC SYSTEM 79 



agent that Increased the chemical action produced by the X- 

 chromosomes would tend to cause the production of fe- 

 males. 



These considerations open the door to many possibilities; 

 since in the chromosomes we are dealing with centers of 

 chemical action, these might well be favored or hindered by 

 various conditions. Changes in the chemical processes might 

 well be induced through the action of certain types of nutri- 

 tion, through temperature changes, through reactions to 

 various stimuli, and the like. Whether such conditions de- 

 termine sex is to be discovered only by observation and ex- 

 periment; there Is nothing In the known method of action of 

 the chromosomes to prevent. 



4. In some organisms the sex of the individual Is known 

 to be determined by the conditions under which it develops. 

 Examples of this are the following: 



(a) In a certain common mollusk called Crepidula plana, 

 If the eggs are kept away from full grown Individuals, they 

 develop into females. But If they are allowed to develop 

 near to older specimens of Crepidula, they become males. In 

 such eggs there is thus a very delicately balanced condition, 

 which may be turned toward either sex by a slight change 

 of conditions.'^ 



(b) In a marine worm known as Bonellia, the female 

 is a large worm with a proboscis. The male on the other 

 hand is very small, and Is parasitic within the body of the 

 female (in the uterus). The eggs develop Into small crea- 

 tures that swim about in the water. If they find a female, 

 they attach themselves to her proboscis, and develop into 

 males. But if they do not become attached to a female, they 

 themselves develop into females. 



Here again there is a delicate balance as to sex: which 

 direction development shall take depends on the external 

 conditions.^ 



