The study of twins offers some hints in the matter of 

 determination of sex. Ordinary human twins are often 

 unhke both as to sex and to physical traits hke ordinary 

 brother and sister. Each originated from a separate ovum 

 and has its own fetal membrane. It is evident, therefore, 

 that external factors, since these were alike, had no influ- 

 ence in determining the sex. 



" Identical twins" are always alike in regard to sex and 

 physical traits. It is believed that they originated from one 

 ovum for they are enclosed in the same fetal membrane. 

 At a very early cleavage the parts become separated and 

 each develops a complete individual. The sex was pre- 

 sumably determined in the fertilized ovum before separa- 

 tion. 



The Free Martin. 



Cattle breeders have observed for a long time that 

 when twins of different sexes are produced the female, the 

 free-martin, is usually sterile. Prof. F. Lillie attributes the 

 phenomenon to the action of the hormones on the two fetus- 

 es which have a constant interchange of blood on account of 

 the development of wide arterial and venous anastomosis. 

 If the twins are both males or both females the hormones 

 produce no harm, but if one is male and the other female 

 the reproductive system of the female is largely suppressed. 



Chapter 17 — IN-BREEDING 



(Consult In-breeding and Out-breeding by East and Jones) 



Considerable difference of opinion exists among breed- 

 ers as to the effect of In-breeding, that is of the mating of 

 closely related members of a family. 



Line-breeding restricts the mating of individuals to a 

 single line of descent. Among plants many self- fertilize, 

 such as wheat, oats, etc., and maintain a vigorous existence. 

 Darwin's experiments with pansy, morning glory, cabbage 

 lettuce, buckwheat and beets (see pages 77-78) went to 

 show, however, that self-fertilization tends to weaken the 

 off-spring and crossing to add vigor and fertility. 



It has been observed, moreover, that "close in-breed- 

 is always detrimental to racing ability. Few winners have 

 been produced by matings of animals more closely related 

 than half-cousins" (Watson, Heredity). 



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