HEREDITY 335 



Further examination showed also that the interstitial cells of the 

 male gonad are differentiated earlier than those in the female 

 co-twin, so that the bloods during the early period of development 

 contain only the male sex hormone. The cause of the diversion of 

 the female sex apparatus toward maleness is thought to be due to 

 the activity of the male hormone prior to the development of the 

 interstitial cells of the ovary of the free martin. It is sometimes 

 stated that human female twins born co-twins v/ith males are sterile, 

 but this is an error. In the development of human twins the em- 

 bryonic circulations do not fuse, as they do in cattle. 



In addition to the principle of sex determination by the sex 

 chromosomes, and the principle of hormonal action in realizing 

 adult sexuality, there are probably other principles that have not 

 yet been established so clearly. In some invertebrate animals, for 

 example in certain small aquatic Arthropoda, it appears that sex 

 development is associated with metabolic rate; when the rate of 

 metabolism of the female is high, her eggs yield a preponderance of 

 male offspring; when it is low, females predominate. If over-ripe 

 frog eggs are fertilized, the majority of the offspring are females. 

 Numerous other examples of sex control that appear to be based 

 on something else than the X chromosome distribution could be 

 cited. So the whole answer to the problem of sex determination has 

 not yet been found. 



Questions of Human Inheritance. Human Interest in he- 

 redity has always been intense and has centred chiefly around the 

 following questions : Can the sex of the offspring be predetermined 

 or controlled? How may family traits be transmitted? May a char- 

 acter acquired by reason of education or training be transmitted? 

 Is disease inherited? Does mating between individuals that are 

 closely related produce degenerate offspring? With the facts of the 

 mechanism of heredity before us and with some knowledge of the 

 behavior of genes in determining the character of the offspring, the 

 subject of human inheritance may be discussed with an appreciation 



