74 I. W. Rowlands 



meanwhile expand; the cytoplasm enlarges and the nucleus 

 becomes spherical in shape. At this stage the corpus luteum 

 has a good blood supply. Changes up to the 12th day are 

 approximately similar in unmated and mated animals, but 

 afterwards, the corpora lutea in the former gradually undergo 

 regressive changes which become intensified when ovulation 

 recurs on the 16th to the 18th day. 



Meanwhile in the pregnant animal, the corpus luteum shows 

 little change until after the 20th day when the luteal cells 

 gradually enlarge and tend to a polyhedral shape. Mitoses 

 are seen more commonly and the gland has a very rich blood 

 supply. This condition of apparent high activity persists 

 until about the 35th day when a very gradual ageing process 

 sets in ; the outline of the cells becomes less distinct, the cyto- 

 plasm is more coarsely granular and vacuoles appear, and the 

 nucleus becomes elliptical in shape. Nevertheless, a small 

 number of the luteal cells appear which have all the characters 

 of those in a young, functional corpus luteum. The amount of 

 fibrous connective tissue increases, which probably serves to 

 maintain the size and shape of the gland. The most striking 

 change which occurs over this period is the restriction of the 

 blood supply due to a progressive closure of the vessels. 



The gland as a whole, however, remains circumscribed until 

 parturition after which it collapses. In so far as a similar 

 change occurs in the corpus luteum of the unmated guinea pig 

 when a new set of follicles rupture, it may well be that the 

 final collapse of the gland at parturition is associated with 

 the recurrence of ovulation at the next oestrus, fifteen to 

 eighteen hours later, rather than from the withdrawal of 

 placental hormones or the evacuation of the uterus. 



Follicular Activity and the Induction of Ovulation 

 in Pregnancy 



At the time of post-partum mating, all follicles other than 

 those destined to ovulate twelve hours later are undergoing 

 atresia, and for the four days after rupture no healthy 



