ACTION OF SEX HORMONES IN FOETAL LIFE 399 



found in the four cases, that I examined, that the twins were 

 dizygotic (in one case both corpora lutea were in one ovary) 

 and that the membranes were fused in the body of the uterus as 

 in cattle. But when injections were made, as was done in all 

 four cases, it was found that the circulation of each individual 

 was entirely closed; the injection mass could not be forced from 

 one side to the other, either through the arteries, or through the 

 veins. Figure 5 gives a faithful representation of one case; it 

 will be observed that the arteries and veins of each side end in a 

 central neutral zone that they do not cross; this zone is no doubt 

 occupied by capillaries, and it is possible that these anastomose 

 from the two sides, though it is uncertain. The other cases were 

 similar, though in one of them a single centrally placed cotyledon 

 received an artery from each side; each artery was accompanied 

 by its own strong vein returning to the same side, which indi- 

 cated that there was little, if any, intermixture of blood in the 

 cotyledon; the starch injection masses, yellow on one side and red 

 on the other were not forced through. 



In the sheep we have, then, all the necessary conditions for the 

 production of sterile free-martins except the actual vascular 

 anastomosis. If the vascular anastomosis should also occur 

 exceptionally, such a condition should be accompanied by ster- 

 ility of the female in the case of heterosexual pairs. This lends 

 probability to the assertion that this condition actually occurs 

 occasionally in sheep. 



The hormone theory thus gives a satisfactory explanation of 

 the occurrence of occasional fertile free-martins in cattle as well 

 as of the usual condition of sterility of the free-martin; and it 

 fits the case of the sheep equally well. As regards other rumi- 

 nants we have unfortunately almost no information. But I 

 have been much interested to find that the famous discoverer 

 of the circulation of the blood, William Harvey, in his "Exer- 

 citationes de Generatione Animalium'' 1651 has some statements 

 on the subject of twin pregnancies in ruminants: thus in Ex. 

 69, p. 487 (Sydenham Society edition, translated by Willis), 

 he says of the deer, ''if the conception be double, one in either 

 horn (of the uterus), each sends its umbilical vessels to its own 



