154 THE PHYSIOLOGY OF REPRODUCTION 



organ is supposed to elaborate. Heape l states that in the case of 

 the rabbit, if the buck is withheld from a doe during several con- 

 secutive cestrous periods, not merely the majority of the older follicles 

 degenerate, but also many of the younger ones, so that the animal 

 is liable to become sterile during the remainder of the breeding season. 2 

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

 tion in immature follicles is lack of sufficient nutriment, or of 

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

 underfed animals, or in animals living' under unsuitable conditions, 

 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. 3 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 Dasyurus, there can be little 

 doubt that the presence of the corpus luteum tends to produce 

 follicular degeneration, or at any rate to inhibit maturation. In 

 the mare, however, Ewart has shown that degeneration 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. 4 



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

 occurrence of coition (see p. 33), 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. O. 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 the other kitten was very small, and 

 apparently about three weeks developed. 



Kuntz, however, has pointed out that the existence of fetuses of 

 varying sizes is not necessarily evidence of superfoetation, since the 

 smaller foetuses may be atrophic. 5 



1 Heape, loc. cit. 



2 Of. Dubreuil and Regaud (C. R. de la Soc. de Bid., vol. Ixvii., 1909), who 

 say that absence of sexual intercourse causes haemorrhage in the follicles. 



3 Ewart, loc. cit. 



4 Ibid. 



5 Kuntz, " Retention of Dead Foetuses in Utero and its Bearing on the 

 Problems of Superfcetation," Anat. Record, vol. xviii., 1920. Cf. Hammond, 

 see below, Chapter XIV., p. 657. 



