196 HEREDITY AND EUGENICS 



examination of corpora lutea (see p. i 'j'j) that 5 to 6 per 

 cent, of ovulations are probably double. When the 

 number of corpora lutea of pregnant rabbits and pigs is 

 compared with the number of embryos, there is found 

 to be always an excess of the former, indicating 

 that some eggs fail to be fertilised after ovulation. 

 Blighted embr3^os are also repeatedly found. They 

 account for some of the human cases of earh' mis- 

 carriages. Such cases are by no means always due 

 to physiological causes, but are often to be explained 

 by inability of the embryo to complete its develop- 

 ment. Lethal factors w^hich kill the embr3^o or 

 inhibit its development beyond a certain point when 

 present in homozygous condition are now^ well known 

 through the work of Morgan and his pupils with the 

 fruit fly Drosophila. It has long been known that 

 onty mice heterozygous for yellow body colour can 

 live, and it has recently been shown that when such 

 3^ellow mice are crossed with each other, 25 per cent, 

 of the embryos — i.e., the homoz^^gous yellows — are 

 atretic and fail to develop. 



Such evidence makes it certain that double ovula- 

 tions are much more frequent than twin births. This 

 will be due, as Davenport points out, to (i) a failure 

 of fertilisation of one egg, or (2) a failure of develop- 

 ment of one egg. Of course, lethal factors may be 

 present in both ^gg and sperm, and their presence 

 will probably account for many cases of sterilit}' in 

 mankind. Thus a woman may be sterile with one 

 husband and fertile with another, or a man may have 

 children by one wife and not by another. Couples 

 desiring children not infrequenth' fail to produce 

 them. In some of these cases the germ cells of both 

 ma}^ contain the same lethal factor, or a combination 

 of factors which are incompatible with development. 

 It is w^ell known that dairy cattle show similar 

 phenomena, a particular bull being sterile with 



