596 THE PHYSIOLOGY OF REPRODUCTION 



of the ovaries of fat cows and heifers. A low condition, 

 especially if associated with exposure to wet and cold, as 

 in the case of cattle wintered in the open air, or of cows 

 which have suckled a large calf or more than one calf, is also 

 a common cause of temporary barrenness.^ Certain other 

 more special causes of sterility are referred to briefly below 

 (p. 606). 



A few years ago the Royal Agricultural Society of England 

 instituted an inquiry into the subject of fertility in sheep. The 

 investigation was conducted by Heape, at whose instigation it 

 was carried out. In the report ^ which was subsequently pub- 

 hshed a comparative account is given of the fertihty of various 

 breeds of sheep chiefly in the south of England in the season 1899. 

 The most fertile breed was the Wensleydale, in which six flocks, 

 consisting of a total of 319 ewes, produced a percentage of 

 177 "43 lambs. The effects of locaUty are discussed, and there is 

 an accumulation of evidence indicating that the character of 

 the district is not without influence on the fertihty of the breed. 

 Thus, Lincoln sheep run on the wolds, Shropshire sheep on a 

 subsoil of new red sandstone, and Hampshire sheep, which are 

 not run upon chalk downs, are shown to be associated statistically 

 with a relatively high percentage of infertihty. The report 

 shows further that the fertihty of a flock depends greatly upon 

 its management, that the quahty and quantity of the food 

 supphed affect the condition of the sheep, and so influence 

 their power to breed, that some seasons are more favourable to 

 fertihty than others, and that sheep-stained pasture (or pasture 

 on which sheep have run for some considerable time previously) 

 is detrimental to breeding stock. 



The present writer has shown ^ that in Scotch Blackfaced, 

 Cheviot, and other Scottish sheep the normal percentage of ova 

 discharged at any single cestrous period is not appreciably in 

 excess of the usual percentage of births at the lambing season. 



1 Wallace (R.), Farm Live Stock of Great Britain, 4th Edition, 

 Edinburgh, 1907. 



^ Heape, "Abortion, Barrenness, and Fertility iu Sheep," Jour. Roy. 

 Agric, Soc, vol. x., 1899. See also Heape, "Note on the Fertility of 

 Different Breeds of Sheep," &o., Proc. Roy. Soc, vol. Ixiv., 1899. 



' Marshall, "The (Estrous Cycle and the Formation of the Corpus 

 Luteum in the Sheep," Phil. Trans., B., vol. cxcvi., 1903. 



