40 INTERNAL SECRETIONS OF THE OVARY 



Horse. Until recently very little accurate information was 

 available as to the oestrous cycle in the mare. Heape (287) made 

 the general observation that the length of the dioestrous cycle 

 is three to four weeks, while Marshall (444) states that the dura- 

 tion of oestrus is about a week, becoming shorter, according to 

 Ewart (i8g), as the season advances. 



Lately, Seaborn and Champy (553), Seaborn (552), and Aitken 

 (4) have brought forward much more data. 



Seaborn gives the length of the dicestrous cycle as twenty-four 

 days, while Aitken found the average to be twenty-two or 

 twenty-three days, with a normal variation from twenty to 

 twenty-five days. The duration of prooestrus and oestrus is three 

 days each, according to the former authors. Copulation takes 

 place only during cestrus. Aitken gives the duration of oestrus 

 as seven days, with normal variation from four to eleven days. 

 Both agree that ovulation occurs towards the end of oestrus. 



The ovary of the horse is rather remarkable for its size and 

 fibrous nature. Ovulation, which is spontaneous at oestrus, 

 takes place from an ovulation fossa. According to Aitken, 

 germinal epithelium covers this part of the ovary, while the rest 

 is covered by peritoneum. Aitken gives the diameter of the 

 mature folhcle as about 6 cms. and of the fully developed corpus 

 luteum as 4 cms. An ovary containing a ripe follicle weighs 

 about 300 gms. and one during dioestrus about 60 gms. 

 Ovulation is not necessarily alternate, and Aitken found a high 

 proportion of double ovulations, often from one ovary. The 

 maturation of the folhcle, and its differentiation from the group 

 of small follicles, seems to occur very rapidly before ovulation. 

 Regressive changes take place in the corpus luteum during the 

 second half of dioestrus. 



The uterine cycle has been roughly described by Seaborn (552) , 

 the central point being the usual proliferation of the mucosa 

 at prooestrus. No vaginal cycle has so far been described 

 and Aitken found that vaginal smears showed no definite cycle. 

 As Marshall states that the mammary gland becomes congested 

 and enlarged during cjestrus, it would appear that a cycle 

 exists in this organ in the non-pregnant animal. 



Sheep. The sheep is a good example of the transition from 

 the monoestrous to the polyoestrous condition. Most wild 



