80 I. W. Rowlands 



active for periods ranging from a few days to about two weeks 

 according to the species. Its life becomes extended in the 

 process of reproduction and it assumes important functions 

 in the maintenance of gestation. The extent to which it is 

 prolonged and its relative importance to the needs of gestation 

 varies greatly from one species to another. Reynolds (1949), 

 in summarizing the evidence for the role of the corpus luteum 

 in the maintenance of pregnancy in different animals, places 

 the guinea pig in an intermediate position between those 

 species in which luteal tissue is required for foetal survival 

 and those in which it plays no part in the second-half of preg- 

 nancy, for the reason that abortion does not invariably occur 

 after oophorectomy in late pregnancy. The observations 

 reported above have shown quite clearly that the corpora 

 lutea persist as well-organized bodies throughout pregnancy 

 in the guinea pig. But mere existence does not necessarily 

 imply retention of functional activity, and in this connection 

 reference must be made to the work of Loeb and Hesselberg 

 (1917) showing that pregnancy was initiated and maintained 

 in some guinea pigs for as long as thirteen days after oophorec- 

 tomy performed as early as the 3rd to 6th day after mating. 

 In others, treated similarly, abortion occurred. That some 

 fundamental change in the function of the corpus luteum takes 

 place at this very early stage of pregnancy is suggested by 

 the renewal of follicular growth and maturation in the guinea 

 pig ovary on about the 4th to 5th day after ovulation. It is 

 difficult in this species, therefore, to establish the precise 

 relationship between the corpus luteum and the maintenance 

 of gestation. 



It would seem, in general, that there are two factors closely 

 associated with the prolongation of the life of the corpus 

 luteum. First, in some species, for example the rat, the 

 duration of luteal activity is prolonged by cervical stimulation 

 at mating. In the guinea pig, however, it has been shown above 

 that mating with a vasectomized male fails to prevent the 

 regression of the corpus luteum which normally occurs on the 

 12th day after ovulation. Secondly, many lines of enquiry 



