612 THE PHYSIOLOGY OF REPRODUCTION 



Two hybrid young ones were produced after a pregnancy lasting 

 twenty-seven days. They were intermediate in size between 

 rats and mice. This is the first record of a cross being obtained 

 between two species so different in size as the rat and the mouse, 

 coitus between them being practically impossible. Furthermore, 

 Iwanoff has successfully employed artificial insemination to 

 obtain hybrids between horses and zebras (a cross which is often 

 difficult to get by the normal method owing to the liability of 

 the animals to refuse service). 



ABORTION 



Abortion is often an important factor in determining a low 

 fertility, but its frequency of occurrence shows a considerable 

 range of variation. 



With women the frequency of abortion to birth at full term 

 is said to be from one in five to one in ten. 1 According to the 

 records of Franz 2 for the maternity hospital at Halle, the per- 

 centage of cases in which abortion occurred was 15*4. Williams 3 

 expresses the opinion that in ordinary private practice every 

 fifth or sixth pregnancy usually ends in abortion, and that the 

 percentage would be considerably increased if one reckoned 

 the early cases in which there is a profuse loss of blood following 

 a retardation of the menstrual period, the actual fact of abortion 

 being often obscured. 



Excepting in the case of sheep, there are no satisfactory 

 data on which to estimate the frequency of abortion among the 

 different kinds of domestic animals, but there can be no doubt 

 that it is of common occurrence, and that it occasions much 

 loss to breeders. For various varieties of English sheep Heape 4 

 found that the percentage of abortion experienced by 300 flock- 

 masters varied from nothing to 23*75, while the percentage for 

 85,878 ewes was 2 '39. The statistics showed that Dorset Horn 

 and Lincoln breeds suffered most from abortion, the losses from 



1 Kelly, loc. cit.' 



2 Franz, " Zur Lehre des Aborts," Hegar's Beitrdge, vol. i., 1898. 



3 Williams, loc. cit. 



4 Heape, "Abortion, Barrenness, and Fertility in Sheep," Jour. Royal 

 Agric. Soc., vol. x., 1899. 



