THE UROGENITAL SYSTEM 



155 



The duration of pregnancy varies widely with different species, 

 and narrowly with individuals of the same species and with 

 the same individual during different pregnancies. Generally a 

 male fetus is carried longer by a unipara than a female fetus. 

 A well-bred precocious animal usually has a shorter gestation 

 period than does the mongrel, scrub or poorly nourished animal. 



The accompanying gestation table, page 153, enables one to 

 calculate when an animal may be expected to give birth to off- 

 spring. Average periods of gestation have been taken in comput- 

 ing the table, that for the mare being 337 days, cow 282 days, 

 sow 113 days, and ewe 148 days. 



Average periods of puberty, estrum, and gestation for differ- 

 ent species of farm animals are given in the following table. 

 These figures have been compiled from a large number of records. 

 In the case of gestation the shortest and longest are extremes 

 which accounts for the wide difference. 



Table Showing Periods of Puberty, Estrum, and Gestation in 

 Farm Animals 



[Puberty 



Estrum 



Gestation 



Animal 



Ave- 

 rage 

 age, 



mo. 



Best 



time 



to breed, 



mo. 



Duration 



Repeats 



Occurs 

 after par- 

 turition 



Short- 

 est, 

 days 



Long- 

 est, 

 days 



Aver- 

 age, 

 days 





10 



10 



6 



6 



24-36 

 15-18 

 9-10 

 18-20 



3-7 days 

 3-48 hours 

 1-5 days 

 1-3 days 



3 weeks 

 3 weeks 

 3-5 weeks 

 13-19 days 



3-17 days 

 28 days 

 3-9 days 

 6-7 mo. * 



287 

 240 

 104 

 140 



419 

 330 

 143 

 156 



337 





282 





113 





148 







* Ewes of the Dorset breed come in heat 7-10 days after parturition. 



The phenomena produced by pregnancy vary in different 

 species. The mare becomes quieter, more docile, and after the 

 third month gradually increases in weight. She usually gains 

 150 to 200 pounds during this time. Fractured bones unite 

 more slowly on account of the drain by the fetus on the mother's 

 osseous elements. Local changes due to pregnancy occur in all 

 the organs of the genital tract but no new functions are acquired. 



Naturally the uterus is most markedly changed. Immedi- 

 ately after conception takes place this organ begins to increase 

 in size and vascularity to accommodate the growing embryo 



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