TYPES OF (ESTROUS CYCLE 37 



Thus, during the time of maximum development of the corpora 

 lutea in the non-pregnant animal, the mammary gland is at its 

 minimum, so that the luteal phase of mammary growth is 

 presumably missing in the normal non-pregnant guinea-pig. 

 Prolongation of the life of the corpora lutea by hysterectomy or 

 by deciduomata formation results in considerable mammary 

 proliferation. 



Cow. The importance of the cow from an agricultural point 

 of view has attracted a large number of workers, among whom 

 may be mentioned Schmaltz (547), Kiipfer (336), Zeitzschmann 

 (645), Murphey and co-workers (468-9), Frei and Metzger (232) 

 and more especially Hammond (265), from whose work the 

 following account is largely compiled. 



The complete dicestrcus cycle in the cow lasts about three 

 weeks, of which oestrus (as determined by discharge of mucus 

 and willingness to copulate) occupies twelve to twenty-four hours. 

 Much variation is, however, found according to breed, age, 

 condition of animal, and time of year. Ovulation is spontaneous, 

 and occurs 0-24 hours after the beginning of heat. Only one 

 ovum is normally produced at each ovulation. The ovaries 

 generally ovulate alternately, though this is not invariable. The 

 rupture of the follicle is accompanied by a small hcnemorrhage. 

 Three days after the beginning of heat the ovaries contain the 

 young corpus luteum, an old corpus luteum from the last cycle 

 (usually in the opposite ovary), and a number of small follicles. 

 Eight days after 'heat,' the old corpus luteum has become 

 insignificant, the new one is fully formed, and a new large 

 follicle has appeared (usually in the same ovary as the old corpus 

 luteum). Ten days later, i.e. three days before the beginning of 

 the next heat, the ovaries are in much the same condition, 

 except that the old corpus luteum has practically disappeared, 

 and slight growth has occurred in the new large follicle. In 

 another two days (one day before heat) degenerative changes, 

 accompanied by decrease in size, have appeared in the recent 

 corpus luteum, and rapid follicular growth is taking place. 



The ovarian cycle in the cow is shown diagrammatically in 

 Fig. 17, taken from Hammond. This diagram suggests that the 

 follicle which will ovulate at the next oestrous period can already 

 be distinguished from the 'reserve' of small follicles at the time 



