52 THE PHYSIOLOGY OF REPRODUCTION 



The elephant in captivity is said to be polyoestrous, but I 

 can find no record of the duration of the dices trous cycle. Since 

 pregnancy is very prolonged (twenty months), the sexual 

 season cannot occur more than once in two years ; that is, if 

 the animals breed. The elephant in the Zoological Gardens in 

 London is stated to have persistent oestrus probably for three 

 or four days. 



CETACEA 



Little is definitely known about the periodicity of breeding 

 in Cetacea. According to Millais, 1 the right whale brings forth 

 in March in every other year, the young being suckled for about 

 twelve months. The humpbacked whales, blue whales, and 

 sperm whales appear to have no regular time for breeding, but 

 Millais says the young of the humpbacked whales are generally 

 born sometime during the summer. Haldane's records, 2 which 

 appear to refer to several different whales, show that foetuses 

 varying in length from six inches to sixteen feet were found 

 in animals captured at the Scottish whaling stations in the 

 summer of 1904. This great variation seems to imply that there 

 is no regular season at which whales copulate, and that very 

 possibly these animals are poly ces trous. Lillie 3 states that two 

 specimens of Balcenoptera musculus were taken off the west of 

 Ireland on July 31, 1909, and that one contained a foetus of one 

 foot in length, while the other had a foetus of five and a half feet 

 in length. Lillie says also that several female rorquals having 

 foetuses of different sizes were captured within a short time of 



published in Quar. Jour. Micr. Science, says that the period of oestrus in mares 

 tends to be shorter the later in the season, and when the food becomes less 

 plentiful and less nutritious all external signs of oestrus disappear. Under 

 favourable conditions, however, mares may become pregnant in winter. 

 Ewart gives the following as the periods of gestation in various Equidae : 

 Asses and zebras, 358 to 385 days ; Prjewalsky's horse, 356 to 359 days ; 

 Celtic pony, 334 to 338 days. In coarse-headed types of horse it is about the 

 same as in Prjewalsky's horse, but in the finer breeds the period is the same 

 as in the Celtic pony. In abnormal cases pregnancy may be unduly pro- 

 longed in mares as in other animals, a mare occasionally going twelve months 

 in foal instead of eleven. 



1 Millais, The Mammals of Great Britain, vol. iii., London, 1906. 



2 Haldane, "Whaling," &c., Annals of Scottish Nat. Hist., April, 1905. 



3 Lillie, " Observations on the Anatomy and General Biology of some 

 Members of the larger Cetacea," MS. still unpublished. 



