108 THE HORSE IN HEALTH AND DISEASE 



drops her foal. The ninth day seems to be tlie usual time for 

 successful mating. If for any reason she does not conceive at this 

 time, the period of estrum returns in twenty-one days and lasts 

 three or four days. It is attended with characteristic signs of 

 nervous or sexual excitement and swelling of the external genital 

 organs. 



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, by means of which it is furnished with 

 nourishment from the mother. At the same time the uterus 

 greatly increases in size and functional activity. 



The fetal membranes consist of the amnion, allantois, and 

 chorion. They are formed at an early date. Details of these 

 structures may be found in works on anatomy, embryology, or 

 obstetrics. 



The period of gestation is the time which elapses between 

 impregnation and the birth of the young. On the average, it is 

 336 days or approximately eleven months for the mare, but varies 

 more in this animal than any other. Cases are recorded where 

 normal foals have been born after having been carried but 300 days. 

 On the other hand, the longest recorded gestation period is 400 

 days. As a rule, a mare carrying a colt-foal is pregnant several 

 days longer than with a filly-foal. A mare with her first foal 

 generally carries it for a shorter period than succeeding ones. 



Signs of pregnancy begin to manifest themselves much later in 

 the mare than in many other animals. Quick subsidence of sexual 

 excitement follows impregnation. Failure of the usual rutting 

 period is often the first indication of pregnancy. The mare be- 

 comes unusually quiet, docile, lazy, and shows a tendency to put 

 on fat. Later the abdomen distends and may become pendulous. 

 At this time a well-marked hollow may be seen in the flank. In 

 advanced pregnancy the fetus may kick about in the uterus, after 

 copious drinks of very cold water by the mother, so as to be dis- 

 tinctly visible. 



Twin pregnancy in the mare results in abortion in about 90 

 per cent, of the cases at the eighth to tenth month. Usually it is 



