MODIFICATION OF THE GERM-CELLS IN MAMMALS 157 



occur without hindering the further development to term of the 

 remaining members. It was also recognized as is known even 

 for the human female that embryos might be absorbed in utero. 

 In the guinea-pig we have found that the absorption of one or 

 more embryos in utero, as is true of partial abortion, may not 

 interfere with the further normal development and birth of the 

 remaining members of such litters. 



When it was realized that these absorptions and abortions of 

 parts of litters were taking place, the necessity arose of definitely 

 detecting each case in order to make the prenatal mortality 

 records approach correctness. A systematic examination was, 

 therefore, begun of every female after being with a male for one 

 month up to within a week or ten days of delivery. 



The female to be examined is allowed to stand on a flat sur- 

 face and the investigator with both hands presses the ventral 

 abdominal wall so as to feel with the fingers the horns of the 

 uterus against the dorsal abdominal wall. With considerable 

 practice the small embryos and placentae may be definitely 

 counted within one or both horns of the uterus. The num- 

 ber of embryos and their position in the two horns of the uterus 

 are noted on the record card of the female. ^ After this initial 

 examination she is reexamined once or twice during the preg- 

 nancy and each time the number and position of the embryos 

 with the date of examination are recorded. The number of 

 young finally born helps to show how nearly correct the exam- 

 inations have been. 



The records now contain several hundred such examinations 

 and show that absorption of embryos may take place not only 

 during early' stages, but after the fetuses have attained consider- 

 able size. The difference between absorption and partial abor- 

 tion may usually be recognized by the fact that the embryo 

 being absorbed may exist for some time as a small lump in the 

 uterus, while the aborted embryo disappears from the uterus and 

 leaves no palpable remains. There are exceptional cases in 

 which the uterus is unusually swollen or congested after the 

 abortion and these on being felt would still seem to contain a 

 partial embryo. The cages of the pregnant females are exam- 



