48 THE PHYSIOLOGY OF REPRODUCTION 



appear to have no regular time for breeding, but Millais says the 

 young of the humpbacked whales are generally born some time during 

 the suinmer. Haldane's records,^ 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 polyoestrous. Lillie ^ states 

 that two specimens of Balcenoptera viusculus were taken off the west 

 of Ireland on 31st July 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 one another. 

 These observations, therefore, are in a general way confirmabory of 

 those of Haldane. 



According to Guldberg and Nansen,^ the porpoise copulates at 

 any time between June and October, the period of gestation being 

 ten months or longer. Meek* states that the testes enlarge 

 enormously in summer. The white-sided dolphin is said to copulate 

 in late summer, pregnancy being about ten months, and the white- 

 beaked dolphin is thought to be similar.^ 



Humpbacked whales and other Cetacea have been described as 

 iiidulging in amorous antics at . the breeding time, 'rubbing against 

 one another and patting one another with their long fins. 



Caknivoea 



In the female of the dog the average duration of the complete 

 oestrous cycle is six months, there being two annual "heat" periods, 

 typically in the spring and in the autumn. It follows, therefore, 

 that the bitch is monoestrous. Bitches belonging to the smaller 

 breeds tend to come " on heat " more frequently than those of the 

 larger varieties. Thus, in Irish terriers, the cycle may recur after 

 four months, though in this breed six months is the more ordinary 

 time.® On the other hand, in Great Danes the duration of the 

 oestrous cycle is often as much as eight months. It would appear 

 that in those cases where " heat " recurs as often • as every four 

 months, this is only when pregnancy is prevented, for more than two 



1 Haldane, "Whaling, etc.," Annals of ficottiak Nat. Hist., April 1905. 



2 Lillie (D. G.), " Observations on the Anatomy and General Biology of some 

 Members of the larger Cetacea," Proc. Zool. Soc, 1910. 



^ Guldberg and Nansen, On the Structure and Development of the Whdlfi, 

 Bergen, 1904. 



* Meek, " The Reproductive Organs of the Cetacea,'' Jow. of Andt., vol. lii 

 1918. 



'■ Millais, loo. eit. 



" Marshall and JoUj-, loc. eit. 



