Heredity of Twin Births. 



77 



Again, of eggs that are fertilized, a certain proportion are 

 aborted at an early age. This fact is striking in mammals also, 

 where occasionally one finds, as John Hammond, 1914, points out 

 and as I can confirm by numerous examinations of the uteri of 

 swine, one or more embryos degenerate at an early stage. Also 

 the medical literature has abundant references to blighted twin 

 fetuses, and the large number of miscarriages under 3 months is a 

 familiar fact. It is probable, since genetic work has revealed a 

 great number of lethal factors, that a large proportion of these 

 atretic embryos is due to such lethal factors. 



The foregoing statements lead to the conclusion that our 

 preconception that twinning is due merely to double ovulation 

 needs revision. It is due to double ovulation combined with 

 some other factors that induces a large proportion of such double 

 ovulations to produce viable twins. 



The other factor is the paternal one. It depends upon the 

 capacity of the male to fertilize all of the eggs ovulated with sperm 

 which contain no lethal factors. Now, both the capacity for 

 complete fertilization (high fecundity) and the absence of lethal 

 factors in the male germ cells are hereditary factors. 



Attention may be called in passing to the fact that in human 

 matings about 10 per cent, are sterile and, among these sterile 

 matings, a certain proportion are physiologically such or at least 

 no imperfections in the reproductive organs of either member of 

 the pair can be detected. 



Summary. — The influence of the male in twin production is 

 determined by the circumstance that twin production does not 

 depend merely upon double ovulation but upon such a quality of 

 the sperm as shall result in a high proportion of fertilization of 

 eggs ovulated and a small proportion of fertilized eggs containing 

 lethal factors. 



