ACTION OF SEX HORMONES IN FOETAL LIFE 379 



the cattle-house kept watch for uteri containing twins, and when 

 they were not of too large size notice was telephoned to the de- 

 partment of Zoology of the University, and our collector, Mr. 

 Adams, went over and brought the specimens to the labora- 

 tory. The collection has been going on for two years and a 

 half, and 55 pairs of twins have been studied. For a long time 

 most of the uteri were received with one or both ovaries missing, 

 but recently special pains have been taken to secure the ovaries 

 also attached to the uterus. A large proportion of the earlier 

 records are therefore incomplete in this respect. 



It was a great surprise to find that nearly all twins of cattle are 

 monochorial; only two complete exceptions have appeared in 55 

 cases. The first case (no. 40) is of great theoretical interest, and 

 will come up for detailed consideration later on. In a very few 

 other cases the connection between the two halves was slight 

 (see cases 8, 9, 10, 24, in table); generally it was broad and 

 strong. At first I had expected to decide the question of mono- , 

 zygotic or dizygotic condition by the monochorial or dichorial 

 state; so that for a considerable period not much attention was 

 paid to the question of the corpora lutea. Later attention was 

 directed to this question and the unexpected result was reached 

 that in all cases in which both ovaries were present each had a 

 corpus luteum. The exact data are: 22 cases in which both 

 ovaries were present; in all of these there was a corpus luteum 

 in each ovary; 7 of these were cfcf,49 9, 10 cf^, and one too 

 young for sex diagnosis; 11 cases in which only one ovary was 

 present, 9 of which had the corpus luteum present, and 2 absent; 

 the two latter were same-sexed, one pair of males, and one of 

 females; they may have been monozygotic, but the missing ovary 

 may have contained two corpora lutea in each case. In 22 cases 

 both ovaries were missing or not recorded. If we consider only 

 those cases, 22 in number,, in which both ovaries were present, 

 there is no exception to the rule that cattle t\vins are dizygotic, 

 using the corpus luteum as evidence for a separate zygote. 

 This is a sufficiently large number to make it certain that the 

 occurrence of monozygotic cattle twins is at least extremely 

 rare. The free-martin condition cannot possibly be interpreted 

 as a result of monozygotic twinning. 



