78 I. W. Rowlands 



at their full size for a longer period. It would seem that the 

 later in pregnancy that ovulation is induced, the more rapid 

 is the growth and the greater is the size attained by the cor- 

 pora lutea. The follicles ruptured on the 35th day form corpora 

 lutea of similar size to those produced during the di-oestrous 

 cycle but unlike the latter they remain at their maximum 

 size, as do the corpora lutea of pregnancy. Histologically, the 

 induced corpora lutea resemble those of the unmated animal 

 for they do not undergo the changes characteristic of normal 

 structures during the 3rd to the 4th week of pregnancy. 



The corpora lutea of pregnancy in the ovaries of the 

 guinea pig in which ovulation was induced on the 35th day 

 (mean D 3 = 7-70:1: 0-68 mm. 3 ) were significantly larger than 

 those (mean D 3 = 5 • 65 ± • 37 mm. 3 ) in ovaries containing a set 

 of corpora lutea induced on the 21st day. Enlargement does 

 not seem to be related to the number of induced corpora lutea 

 co-inhabiting the ovaries, although in one animal (IOP/19) 

 it did not occur when only one of the latter structures was 

 present. 



Functional Activity of the Induced Corpora Lutea 



The inhibition of follicular activity 



Six pregnant guinea pigs were injected with chorionic 

 gonadotrophin and killed 24 to 96 hours later. All follicles 

 present at the time of injection which failed to ovulate became 

 heavily luteinized and were recognizable as small luteal cysts 

 containing an egg. No normal vesicular follicles were found 

 in ovaries up to 96 hours after injection, but in other guinea 

 pigs killed after eight days they were as plentiful as in a 

 normal animal after mating. 



Capacity to sensitize the uterus to decidual reaction 



A few attempts were made to test the capacity of induced 

 corpora lutea to sensitize the uterus in this way. Unilateral 

 pregnancy was established by ligation of the left uterine horn 



