EMBRYOIX)GY, IMPREGNATION, CONCEPTION. 37 



to work forward. They enter the womb, usu- 

 ally in large numbers, and some penetrate into 

 the Fallopian tubes where, according to the 

 best authorities, the first stage of impregnation 

 takes ])lace. The egg seems to have a strong 

 attraction for the spermatozoa. Surrounding 

 the egg is a soft envelope which is readily 

 pierced by the comparatively hard head of the 

 spermatozoon, probably by several. One alone, 

 however, forces its way into the center of the 

 egg^ his tail is broken off and no more are al- 

 lowed to enter. This forms what is termed the 

 male pro-nucleus. In the egg at about the 

 same time the female pro-nucleus is formed and 

 those two moving together unite and complete 

 the process of impregnation. In the egg there 

 is a yolk which, after fertilization, is first de- 

 veloped to greater proportions than when im- 

 pregnation took place, supplies sustenance for 

 the embryo and later is absorbed. When it 

 is considered that there are no two things in 

 animal life exactly alike, and when it is known 

 that only one of hundreds, perhaps thousands, 

 of spermatozoa actually fertilizes the egg, it is 

 easily recognized how vast and uncertain the 

 problem of heredity really is. 



Development of the fetus in the womb of the 

 mare has been more or less accurately worked 

 out. Description of the changes noted, how- 

 ever, belongs to the domain of the veterinarian 

 rather tlmn of tho breeder. It is pertinent to 



