FERTILITY 597 



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 flock- 

 masters, and which is proved very clearly by Heape's statistics. 

 And since ewes which are constitutionally 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 un- 

 common in sheeps' ovaries, the process of atresia appearing to 

 set in most commonly in 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. 156). There can be little doubt that an excessive follicular 

 degeneration results usually 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 sup- 

 posing 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. 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. 



