PHASES IN THE LIFE OF THE INDIVIDUAL 717 



Other changes, depending probably on the degeneration of the 

 ovaries, are the assumption of certain of the secondary male 

 characters. These are apparently more marked in some animals 

 than they are in the human species, and have already been men- 

 tioned in dealing with the internal ovarian secretions (p. 340). 



The ages at which domestic animals cease to breed have been 

 only imperfectly determined since they generally die before reaching 

 their climacteric. Mares have been known to produce young beyond 

 thirty years, 1 sheep up to twenty, and cats to fourteen, but they may 



FIG. 184. Section through uterine mucous membrane of woman of 

 sixty. (From Sellheim.) gl, Glands. 



cease somewhat earlier and yet maintain a healthy existence for a 

 few years. 2 Kirkham 3 says the white mouse stops reproducing at 

 eighteen to twenty-two months after having twelve to sixteen litters. 

 The menopause in the white rat occurs at the age of fifteen to 

 eighteen months. 4 



1 Wood (W. A.) found that out of 1216 thoroughbred mares recorded in 

 the earlier volumes of the General Stud-book, one bred at 33, two at 30, four 

 at 29, seven at 28, and seventeen at 27. He draws the conclusion that in actual 

 practice the mare generally continues to breed as long as she lives (" Note on 

 the Breeding Age of Thoroughbred Mares," Vet. Jour., December 1921). 



2 Fleming, Veterinary Obstetrics, 3rd Edition, by Craig, London, 1912. 



3 Kirkham, loc. cit. 



4 Donaldson, loc. cit. 



