18 Pork Prodvction 



and again before the litter is weaned. The natural breed- 

 ing seasons seem to be the early summer or late fall, 

 although a vigorous sow when gaining will come in heat 

 at other times, even in the hottest summer months. 



Researches by L. L. Lewis of the Oklahoma Experiment 

 Station on the vitality of germ-cells indicate that the 

 ripened eggs or ova are not discharged from the ovaries 

 until the latter end of the heat period.^ These studies 

 also revealed that the sperm-cells of the boar do not, as 

 a rule, retain their vitality in the body of the sow for a 

 longer period than sixteen hours after service. These 

 observations would seem to support the theory that 

 successful impregnation is most certain when the sow is 

 bred in the latter stages of the heat period. So far, 

 general observations in practice, however, have shown 

 no advantage for early or late breeding so far as either 

 may affect complete fertilization or the size of the residting 

 litter. 



Length of gestation period. 



The time elapsing between breeding and farrowing is 

 known as the gestation period. During this time each 

 of the fertilized eggs develops into an embryo pig, and 

 with the completion of the pigs' embryonic development, 

 birth takes place. Normally, the gestation period is just 

 long enough to make possible full embryonic development 

 of the pigs and to accommodate those physiological adjust- 

 ments in the sow which are preliminary to parturition 

 or farrowing. 



Gestation tables which have been worked out for the 

 convenience of breeders are calculated on the basis of 

 112 days, or 16 weeks, as the time elapsing between breed- 

 • L. L. Lewis : Bull. 96, Okla. Exp. Sta. 



