THE UROGENITAL SYSTEM 151 



age, but should not be placed in regular service until he is three 

 or four years old. The filly is usually sexually mature when 

 one year old, but should not become a mother until she reaches 

 the age of three years, or four if she is not precocious. This 

 means that it is safe to breed her at two years of age. There is no 

 experimental evidence to support the belief that the long con- 

 tinued practice of breeding immature females will result in de- 

 creasing the size of the breed or in the production of more refined 

 types. The capacity to develop to a certain size at maturity 

 is a hereditary tendency and is not influenced in any way by the 

 breeding of young animals. 



Estrum, heat, rut, are expressions commonly applied to the 

 period during which the female exhibits a. desire to mate. Ordi- 

 narily mating will be permitted only during the period of estrum. 

 Estrum usually occurs in the mare between the fourth and the 

 twelfth day after her foaling. The ninth day seems to be the 

 usual time for successful mating. If for any reason she does not 

 conceive at this time, the estrual period returns every twenty-one 

 days and lasts three or four days, except during the winter in 

 some cases. It is attended with characteristic signs of nervous 

 or sexual excitement and swelling of the external genital organs. 



When an animal has been safely bred she is said to have con- 

 ceived. Horsemen usually present the mare to the stallion ten 

 to fourteen days after service, also on the twenty-first day, to 

 determine whether conception has taken place; if she refuses to 

 take the stallion it is generally safe to conclude that she has 

 "settled," conceived, or become impregnated. 



Impregnation starts a role of rapid changes that are of great 

 interest. The fertilized ovum quickly develops by the process of 

 cell division, from a one-cell structure into a mass of cells, which 

 arrange themselves in a definite manner to form three layers — 

 the epiblast, the hypoblast, and the mesoblast. It becomes 

 attached to the mucous membrane of the uterus through the 

 formation of the placenta. At the same time the uterus greatly 

 increases in size and functional activity. 



The fetus is an entirely independent organism. Its circulation 

 and metabolism are quite separate and distinct from that of the 

 mother. As it cannot use its lungs to purify its blood, this func- 

 tion is performed by the mother. The exchange of gases takes 

 place through the placenta. There is no direct connection between 



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