148 CARNEGIE INSTITUTION OF WASHINGTON. 



summer Dr. George W. Corner, jr., of the Johns Hopkins Medical 

 School, assisted by Mr. Clyde E. Keeler, of Denison University, 

 extended this statistical study into several hundred pregnant uteri. 

 It appeared from all of these counts that nearly 98 per cent of the 

 eggs ovulated enter the Fallopian tubes. Only about 80 per cent, 

 however, are developing during the second month, and only about 70 

 per cent develop to the last third of pregnancy. Probably not more 

 than two-thirds of the eggs ovulated result in pigs born alive. Just 

 what happens during the first month of pregnancy which should cause 

 the failure of so many eggs to develop is yet uncertain. Probably a 

 certain proportion of the eggs are not fertilized. It is certain that of 

 the fertilized eggs a certain proportion, which may lie between 10 and 

 20 per cent, proceed along their development to different points and 

 then die. Similar blighted fetuses have been commonly found by 

 obstetricians, and scores of cases of blighted twin fetuses are recorded 

 in the literature. Also, as is well known, miscarriages and stillbirths 

 are fairly common among humans, of which an important cause is 

 apparently sheer inability to continue development because of the 

 internal weakness of the embryo. Children who are born are already 

 a selected group from among those whose development has been 

 initiated. Such incapacity for development has been observed by 

 geneticists in a large number of cases, and the category of lethal 

 factors which inevitably prevent further development is now well 

 recognized. Such lethal factors probably correspond to gross variation 

 in essential visceral organs and run parallel to such gross variations 

 of external organs as cleft palate, microphthalmia, and the absence 

 of appendages. Now, such lethal factors may be brought into the 

 zygote by the egg alone, by the sperm alone, or by both. They are 

 not found in all germ-cells; it may be only in a small proportion of 

 them. When they occur in the gametes of both consorts, small families, 

 with some feeble children, may be expected; but when absent in the 

 germ-cells of both parents, then, in a good environment, the fertilized 

 egg will develop vigorously, with good prospects of reaching maturity. 

 Now, it is in such families that any tendency toward double ovulation 

 will be expressed in the production of healthy twins. This accounts 

 for the long-known statistical fact that the proportion of twins is greater 

 in highly fecund families than in those that produce few offspring. 

 Also, among humans there is probably a frequent failure of fertiliza- 

 tion of both eggs, resulting in a development of only one of a potential 

 pair. The preceding considerations make it clear where the male 

 factor enters in twin production; for the father, as much as the mother, 

 determines whether both of a pair of simultaneously ovulated eggs 

 shall be fertilized, and whether or not they shall receive lethal factors. 

 The statistical studies made on the relation between number of 

 corpora lutea and number of embryos have been greatly extended by 



