CHANGES IN THE OVARY 159 



held from a doe during several consecutive oestrous periods, 

 not merely the majority of the older follicles degenerate, but 

 also many of the younger ones, so that the animal is Uable to 

 become sterile during the remainder of the breeding season.^ 



There can be little doubt that the more usual cause of de- 

 generation in immature follicles is lack of sufficient nutriment, or 

 of nutriment of the requisite kind. It is usually to be observed 

 in under-fed animals, or in animals hving under unsuitable con- 

 ditions, but it also occurs in very fat animals. Ewart states 

 that follicular degeneration tends to occur in mares leading a 

 semi- wild life in winter.^ Probably it is least common in animals 

 which are in a good thriving condition, but further investigation 

 is urgently needed before these points can be decided. 



SUPERFCETATION 



In the majority of Mammals, as in Basyurus, there can be 

 little doubt that the presence of the corpus luteum tends to 

 produce follicular degeneration^ or at any rate to inhibit matura- 

 tion. In the mare, however, Ewart has shown that degeneTation 

 does not generally take place during early pregnancy, so that 

 if a mare aborts (a common occurrence with this animal) ripe 

 ova are available for fertilisation, and pregnancy can be started 

 anew without delay. ^ 



If ovulation takes place during pregnancy, and if, owing to 

 the occurrence of coition (see p. 51) the ova become fertilised, 

 the phenomenon of superfoetation may take place — that is to 

 say, foetuses of different ages may be present in the same uterus 

 — ^but this condition is of course abnormal, though it has been 

 known to occur in several animals. Thus, Mr. W. 0. Backhouse 

 has informed me of a case of a cat which experienced heat and 

 underwent coition after being pregnant for six weeks, and three 

 weeks later produced five kittens, four of which were of the 

 normal size and were obviously born at full time (dating from 

 the heat period prior to the beginning of pregnancy), whereas 



' Gf. Dubreuil and Regaud (G. R. de la Soc. de Biol., vol. Ixvii., 1909), 



who say that absence of sexual intercourse causes haemorrhage in the 

 follicles. 



' Ewart, loc. cit. ' Ibid. 



