42 THE PHYSIOLOGY OF REPRODUCTION 



in the case of so many other Rodents.^ The period of gestation 

 is about sixty-two days, an unusually long time for so small an 

 animal, being more than twice as long as the gestation period of 

 the rabbit. As a result the newly born guinea-pig is well 

 advanced in development, and able to feed for itself, instead 

 of being dependent on its mother's milk. 



In most male Rodents the testes undergo a periodic increase 

 in size and descend into the sessile scrotum at the beginning 

 of the season of rut, after which they become smaller again, and 

 are withdrawn into the peritoneal cavity. In the Leporidse, 

 however, and in some other species, the testes are not so re- 

 tracted, but remain throughout the year in the scrotal sacs.^ 



Ungulata 



This order contains several examples of animals which are 

 almost certainly monoestrous in a state of nature, but are 

 polycestrous in captivity or under domestication. In the latter 

 case the increase in sexual capacity appears to be due partly to 

 the inherited eiiects of domestication, and partly to the direct 

 influence of a more favourable environment. 



For example, the sheep presents a complete gradation from 

 the apparently monoestrous condition of some wild species to 

 the extreme degree of polyoestrum, which is reached by certain 

 of the more domesticated breeds.' 



The Barbary wild sheep (Ovis tragela'phus) in the Zoological 

 Society's Gardens is monoestrous, breeding only once annually.* 

 The same is stated to be the case with the Burrhel sheep 

 (0. hunhel), although the moufflon (0. musimon) in captivity may 

 experience two or more recurrent dicestrous cycles in an annual 

 sexual season.* It would seem, however, from the account 

 given by Lydekker * of the breeding habits of 0. musimon, as 



' Sobotfca, " tJber die Bildung des Corpus Luteum beim Meerschweinchen," 

 Anat. Hefte, vol. zxxii., 1908. 



" Owen, On the Anatomy of Vertebrates, voL iii., London, 1868. 



» Marshall, "The (Estrous Cycle, &c., in the Sheep," Phil. Trans. B., 

 vol. oxcvi., 1903. 



* Heape, loc. cit. 



= I am indebted to Mr. F. E. Beddard, Prosector of the Zoological Society, 

 for this information. 



^ Lydekker, Wild Oxen, Skeep, and Goats of All Lands, London, 1898. 



