82 I. W. Rowlands 



is provided only when a certain threshold amount of decidual 

 development or uterine distension has occurred. 



It has been made clear that not only is the life of the corpus 

 luteum prolonged in the pregnant guinea pig but that it is 

 also significantly enlarged. Are these two features different 

 expressions of a response to the same stimulus or are they 

 controlled independently? It is well known that in many 

 species prolongation of luteal activity is not accompanied by 

 enlargement of the gland, which may be an indication that 

 these characters are controlled separately. The observations 

 made on a set of corpora lutea resulting from ovulation induced 

 at different stages during pregnancy confirm this view for, 

 although these structures never attain the size of the normal 

 corpora lutea of pregnancy, they have a longer life than those 

 of the unmated animal. 



Summary 



The corpus luteum of the guinea pig reaches maximum 

 size on about the 12th day and regression, which sets in 

 immediately, is accelerated when ovulation recurs four to six 

 days later. Sterile mating does not prolong its life-span. In 

 pregnancy, the corpus luteum grows at the same rate for 

 eighteen to twenty days and is maintained at maximum 

 size until parturition, but histological evidence of ageing 

 occurs in mid-pregnancy. Regression is very rapid after 

 parturition. 



Follicular growth is resumed on the 4th to 5th day after 

 mating and mature follicles are found at all times after the 8th 

 or 10th days. Ovulation does not occur spontaneously but 

 may be induced by chorionic gonadotrophin (50 i.u.) injected 

 at any time after the 8th day. The number of follicles that 

 rupture and the size of the corpora lutea that are induced are 

 greater in late than in early pregnancy. Those produced on 

 the 53th day grow to the size of the corpora lutea in the 

 unmated animal. They never reach the size of the co-existing 

 corpora lutea of pregnancy but, like the latter, they are 



