THE BREEDING SEASON 63 



sire of both twins are probably much more common than 

 is generally believed. In such cases it is difficult to deter- 

 mine whether the twins foaled at the same time are the 

 result of the fertilization of two eggs ripened at one and 

 the same heat period, or whether the eggs have been ripened 

 at different periods some distance apart. 



A case of this kind has been reported by J. A. Finley 

 of Troy, Missouri. In this case the mare was bred to a 

 jack, and twenty-one days later she was found in heat 

 again and was again bred to the same jack. At the end 

 of a few months she aborted, losing twin mule colts. 

 One of these was much better developed than the other. 

 It seemed clear that the two colts were from different 

 periods of heat, in this instance, twenty-one days 

 apart. 1 



The following examples are probably to be regarded 

 as cases of superfcetation : A sow belonging to O. 

 Young of Hopkins, Missouri, gave birth to three pigs. 

 The mother nursed them for four or five days, when she 

 weaned them. Three weeks later the sow gave birth to 

 eight pigs, six of which lived and became thrifty young 

 hogs. 



A young ewe owned by A. Cassity of Linneus, Missouri, 

 gave birth to twin lambs on February 13th. About 

 six weeks later on March 30th, she gave birth to a third 

 lamb. See Plate I. 



62. Recurrence and duration of the oestrum. The 

 occurrence and duration of heat are influenced by age, 

 species, food supply, season, heredity and other conditions. 

 The heat period persists in the domestic animals for one 

 to fifteen days. 2 The duration of the heat period is 



1 From letter to Geo. F. Nardin, dated June 24, 1912. 



2 Hill, "Bovine Medicine and Surgery." 



