FERTILITY 633 



The present writer has shown ^ that in Scotch Blackfaced, 

 Cheviot, and other Scottish sheep the normal percentage of ova 

 discharged at any single oestrous period is not appreciably in excess 

 of the usual percentage of births at the lambing season. It would 

 seem probable, therefore, that a scarcity of twin births at lambing 

 time is the direct consequence of an abnormally low number of ripe 

 follicles in the ovary at tupping time {i.e. during the sexual season). 

 A low percentage of twins is generally associated with barrenness, a 

 fact which is recognised by flockmasters, and which is proved very 

 clearly by Heape's statistics. And since ewes- which are con-' 

 stitutionally barren are a rarity, there can be little doubt that 

 infertility among sheep ■ is due ordinarily to an absence or great 

 scarcity of follicles available for ovulation at tupping time. 



Scarcity of mature follicles must itself result either from a 

 retardation in follicular development or from an unusual tendency 

 towards follicular degeneration whether occurring shortly before the 

 sexual season or at some previous period in the animal's lifetime. 

 Atretic or degenerate follicles are by no means uncommon in slieeps' 

 ovaries, the process of atresia appearing to set in most commonly iii 

 follicles which have reached a size varying from about one-eighth 

 to one-half the dimensions of the mature follicle. It may set in, 

 however, at practically any stage of development and probably in 

 the so-called primordial follicle (see p. 151). There can be little 

 doubt that an excessive follicular degeneration results iisually from 

 an insufficiency of stimulating power at the disposal of the ewe. 



That a relative scarcity of ripe follicles in sheeps' ovaries at the 

 sexual season is due to retardation of development is a conclusion 

 which is based on inference rather than observation, for little, is 

 known regarding the actual rate of growth of the Graafian follicle. 

 Nevertheless, there is every reason for supposing that the processes 

 of growth and maturation can be very largely influenced both by 

 insufficiency of food supply on the one hand and by artificial 

 stimulation on the other, as has been shown for other, animals. 

 This fact has been recognised for years past by certain individual 

 flockmasters who have consistently practised the methods of 

 "flushing" or artificially stimulating their ewes by means of an 

 extra supply of special food at the approach of the tupping season, 

 but no precise records of the effects of this process had been 

 published until lately, when the Highland and Agricultural Society 

 of Scotland undertook an investigation upon this subject. 



In the report which has since been issued,^ and which contains 



1 Marshall, " Tlie QEstrous Cycle and the Formation of the Corpus Luteum 

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



2 Marshall, " Fertility in Scottish Sheep," Trans. Highland and Agric. Soc, 

 vol. XX., 1908. See also Froc. Roy. Soc, B., vol. Ixxvii., 1905. 



