FERTILITY 635 



and that if this is omitted the sheep tend to be less fertile than if 

 they had never been subjected to flushing. 



It has already been mentioned (p. 364) that the practice of 

 flushing tends to hasten the sexual season, the sheep coming "on 

 heat" sooner than they would otherwise. The result must be 

 ascribed to a general increase in the ovarian metabolism consequent 

 upon the stimulating power of the special food supply. Conversely, 

 it has been shown that in ewes which are poorly fed the sexual 

 season is often retarded, and the fertility of the flock reduced. So 

 also the occurrence of a snowstorm, or other unfavourable climatic 

 condition, occurring during tupping time will cause a corresponding 

 scarcity of twin births in the following lambing season. There can 

 be little doubt, therefore, that the conditions which exist during 

 tupping time are largely responsible for controlling the fertility of 

 the flock, and that favourable conditions tend to promote the more 

 rapid growth and maturation of the follicles in the ovary, and cause 

 a greater number to discharge their ova during the cestrous periods. 



It would appear also that in ordinary agricultural practice the 

 condition of the ewe is a far more important factor in determining 

 the number of twin births than that of the ram ; but it is obvious 

 that the number of ewes which one ram can serve successfully must 

 depend upon the degree of vigour possessed by him, and that the 

 keeping of a ram which is partially sterile and yet is turned out 

 to serve ewes must result in a reduction in the number of Iambs (see 

 below, p. 637). 



The effects of nutrition upon the production of ripe follicles in 

 guinea-pigs have been studied by Leo Loeb ^ who states that under- 

 feeding if very pronounced prevents maturation in all cases, and 

 causes atrophy before the follicles have reached medium size. A 

 premature solution of the epithelial cells is brought about in this 

 way, but the connective tissue of the ovary is more resistant to the 

 effects of underfeeding. The cells which are furthest from the 

 blood-vessels suffer first. The epithelial cells, however, continue to 

 multiply as long as they survive, and this is ascribed to a normal 

 growth stimulus which emanates from the ■ ovum. The result of 

 poverty of nutrition is more marked in the ovaries of young animals 

 just as the general effect is greater, but a " hypotypical " condition 

 can be induced even in old guinea-pigs. In an extreme case of 

 hypotypical ovaries the connective tissue separating the follicles had 

 become affected, being undeveloped or lacking, so as to result in a 

 union of follicles and a consequent polyovular condition.^ Loeb 



' Loeb, "The Experimental Production of Hypotypical Ovaries through 

 Underfeeding," Biol. Bull., vol. xxxii., 1917. 

 2 See footnote, p. 121. 



