No. 631] 



HUMAN TWINS 



127 



there seems to be no failure of fertilization, except 

 at the end of the series. 

 9 children— 4, 2, 2, 2, 2, 2, 2, 2. 

 10 children— 3, 1, 4, 2, 1, 2, 2, 2, 2. 

 9 children— 1, 2, 3, 4, 3, 2, 2, 3 ; born between 1825-1845. 



One gets the impression that the normal interval be- 

 tween births, assuming all eggs to be fertilized, is about 

 2 years. The frequent intervals of 3, 4, 5 and even more 

 years probably correspond to failure to fertilize, although 

 they may be due to miscarriages or even in some cases to 

 prolonged absence of the husband. In view of the fact, 

 however, that we have to do here with a prevailingly 

 rural population, chiefly farmers and millers in central 

 New York State, the latter contingency is improbable. 



The failure of fertilized eggs to complete their develop- 

 ment is a real factor that must be taken into account. 

 Attention has been called to the importance of this factor 

 by John Hammond (Journal of Agricultural Science, VI, 

 1914) who has studied fetuses of rabbits and pigs and 

 finds among them many degenerating individuals. Thus 

 the number of degenerating fetuses in a large number of 

 uterine horns examined varied from to 19 per cent. I 

 can confirm these results by observation made upon the 

 uterus of a sow (No. 3) in which there were 2 corpora 

 lutea in the left ovary and 5 in the right. In the left horn 

 of the uterus there was a well-developed embryo 8 mm. 

 long and one, evidently blighted, of 4 mm. The outlines 

 of the latter embryo were highly abnormal and shrunken. 

 The right horn of the uterus contained one embryo, 25 

 mm. long, a second 9 mm. long, and a third 6 mm. long. 

 Thus with 7 corpora lutea in the ovaries, there were only 

 5 embryos found, of which one was completely blighted, 

 another at 6 mm. length would probably soon have ceased 

 development and two others at 8 and 9 mm. were far be- 

 hind the best developed embryo, already 25 mm. long. 



Work on yellow mice, of which the yellow X yellow 

 matings give rise to 25 p. c. atretic embryos, and the far 

 more extensive experience of Morgan with lethal factors 

 in Drosophila, indicate that failure of development is a far 



