No. 631] 



HUMAN TWINS 



129 



have been ovulated in each case, but one of the pair has 

 failed to develop, either through failure of fertilization or 

 early blighting. 



Now the lethal factors show their influence first in cer- 

 tain combinations, just as in the matings of yellow X 

 yellow mice. The 1 of the embryos which die are those 

 which are derived from germ cells containing the genes 

 for yellow, whereas the other f may develop fully. So 

 we concude that among humans the cases of twin-repeat- 

 ing fraternities are those in which there are no or few 

 lethal factors in the germ cells, so that there is a maxi- 

 mum fertilization and development of the eggs laid. 1 In 

 the case of families comprising only one pair of twins, 

 combined with a number of single births, it is probable 

 that in other cases there had been a double ovulation but 

 one of the pair had failed to develop. The additional fact 

 to be taken into account is that twins are found in a higher 

 ratio in large families than in small ones. Large families, 

 however, connote high fertility of the male as well as the 

 female. From all these facts we reach the conclusion 

 that families which readily produce twins do so not only 

 because in the mother the eggs were laid in pairs, but also 

 because in the father the sperm is active, abundant and 

 without lethal factors, so that the number of eggs fer- 

 tilized and brought to full term approaches a maximum. 

 To repeat, such fathers, experience indicates, belong to 

 strains which are exceptionally fertile and in which twins 

 are repeatedly produced both along male and female lines. 

 Thus it comes about that the fathers of twins are about as 

 apt to belong to twin-producing strains as mothers of 

 twins and that twinning depends on constitutional — 

 hereditary— factors on both sides of the house. 



*F. II. A. Marshall (1910), "Physiology of Eeproduetion, ' ' p. 618, rec- 

 ognizes that certain abortions in sheep "may be due to a want of vitality 

 on the part of the developing embryo. " Similarly gynecologists recognize 

 that a part of the 10 per - cent, of barren marriages^ and many of the early 



physiology. 



