TYPES OF (ESTROUS CYCLE 41 



species, according to Marshall (444), are probably monoestrus, 

 having only one oestrous period in the short breeding season. In 

 captivity, however, and particularly under conditions of domes- 

 tication, two or more dioestrous cycles appear during the breed- 

 ing season, while the Australian Merino is said to experience 

 an unbroken series of dioestrous cycles throughout the year in 

 the absence of pregnancy. In Great Britain the breeding season 

 lasts from two to three months. According to Marshall the length 

 of the cycle is about fifteen or sixteen days, though this doubt- 

 less varies according to breed and nutrition . Other authors have 

 given from two to four weeks as the length of the cycle. 

 Neither vaginal nor mammary cycles have been described in the 

 sheep, but Marshall (440) has given a full description of the 

 changes taking place in the non-pregnant uterus. These changes 

 consist essentially of the phases of rest, growth, destruction and 

 regeneration, such as have been described in the dog. 



During the growth phase the mucosa increases in thickness, 

 the stroma undergoes cell division, and uterine congestion 

 begins. The period of destruction is characterized by the 

 breaking dow^n of some of the capillaries and extravasation of 

 blood into the stroma. Bleeding into the uterine cavity does not 

 usually occur. The extra vasated blood usually remains under 

 the endometrium, giving rise to pigmentation. This period of 

 destruction, corresponding to the end of prooestrus, is followed 

 by a period of recuperation. Subsequently the uterus returns to 

 the condition of rest. 



Pig. The duration of the dicestrous cycle in the sow varies 

 from two to four weeks, but is usually three weeks. Struve (604), 

 in a statistical investigation, found the average to be 20-66 ± -205 

 days. The external signs of heat are excitement of the animals, 

 and swelling of the vulva, from which there may be a slight 

 (possibly sanguinary) discharge. According to Corner (122), 

 however, the blood is of vulval rather than internal origin. 

 CEstrus lasts about three days. 



The ovarian cycle in the pig has been described in detail by 

 Corner (122). About three days before the onset of oestrus, the 

 follicles due to ovulate undergo rapid growth and finally attain 

 a diameter of 8-10 mms. Ovulation occurs towards the end of 

 oestrus according to Lewis (362), but Corner and Amsbaugh 



