192 PHYSIOLOGY OF FARM ANIMALS [CH. 



average sized litter consisting of four or five jjups while the 

 number is often fewer. 



The number of lambs produced may be increased by the pro- 

 cess of ' flushing ' the ewes, that is, by giving them extra feeding, 

 cake, corn or tvirnips for a few weeks before tupping time, this 

 practice not merely increasing the number of ripe follicles available 

 for ovulation at any one time, but also having the effect of hasten- 

 ing forward the sexual season. After tupping is over extra 

 feeding is no longer of use in increasing fecundity since the number 

 of developing young has already been determined. 



It is clear that, speaking generally, the female is a more im- 

 portant factor than the male, since whereas the number of ripe 

 ova available for fertilisation is very limited, the number of 

 spermatozoa entering the female passages is excessive. 



In the sow Hammond has shown that whereas a very large 

 number of ova may be fertilised, some of these may only undergo 

 a very limited degree of development, but it is uncertain what 

 precise factors are responsible for this failure. It is possible that 

 some of the ova possess an insufficient vitality from the outset, 

 and this may possibly be the result of inbreeding which is believed 

 to tend towards infertility. Thus Heape states that Dorset Horn 

 ewes will sometimes produce lambs when put to Hampshire Down 

 rams but will not breed when served by rams of their own variety. 

 It is possible that the fertilised ovum is endowed with a superior 

 vitality by having conjugated with a spermatozoon belonging to 

 a different breed. 



Fertility like other characteristics can be inherited, and a ram 

 which was one of twins and consequently of fertile stock may 

 transmit this capacity to female progeny. It is improbable, how- 

 ever, in view of what has been said above, that the male parent 

 can generally be responsible for the size of the litters produced by 

 the females served by him. Nevertheless fertility may be trans- 

 mitted through the male in just the same way as the milking 

 capacity of a dairy cow may be transmitted through the male 

 progeny to the next generation of females. 



The reproductive period in the life of the female begins at 

 puberty when the generative organs develop, the first batch of 

 ova become mature, and the first oestrous cycle commences: it 

 ends at the menopause or climacteric when the generative organs 

 become atrophic and the cycle closes. The mare begins to breed 



