Hermaphrodites 495 



from the male twin had entered into the female and had started 

 some of its structures to develop in a male direction. The result 

 of the two types of hormones in the developing female is a 

 sterile female with some decidedly male characters. Even though 

 the later-developing female hormone diffuses into the male, the 

 male animal has developed so far at the time that the female 

 hormone has no apparent effect. The situation responsible for 

 the circulation of blood from one twin into the other is peculiar 

 to cattle. Lillie showed that in cattle the twins develop a com- 

 mon circulation so that the female twin is actually supplied with 

 blood that contains the male hormone. In other animals twin- 

 ning is not accompanied by such anastomisis of the fetal circu- 

 latory systems, and no freemartin is produced. 



Environmental Control 



In Crew's hen, the removal of the ovary by disease controlled 

 the sex of the individual. Similar results may be brought about 

 experimentally during early development by the artificial re- 

 moval of sex organs and the grafting of organs of the opposite 

 sex and by artificially injecting into an animal the sex hormones 

 of the opposite sex. Such methods show that the environment 

 plays a considerable part in determining the sex of an individual 

 that is potentially both a male and a female. A classical ex- 

 ample of the effect of environment is illustrated by the marine 

 worm Bonellia. The young individuals are potentially both male 

 and female. When they are in the young larval stage, they swim 

 about freely. If one comes into contact with an adult female 

 and becomes attached to it, that larva develops into a male; 

 but if a larva fails to locate on a female and merely undergoes 

 its later development on the bottom of the ocean, it becomes a 

 female. Sex develops comparatively late in these animals and 

 according to the environment of the animal during its later larval 

 development. 



Hermaphrodites 



Hermaphrodites, as we pointed out in Chapter 4, are indi- 

 viduals that produce both male and female gametes. Thus such 

 an individual is both male and female at the same time. This 

 situation is found as a normal feature of the life cycle of some of 

 the lower animals, such as the common earthworm. It may 



