Essentials of Animal Breeding. 5 



each side, situated in the abdominal cavity. Connecting the ovaries 

 with the uterus are the Fallopian tubes. The uterus is the womb, 

 the chamber in which the young fetus develops until it is ready for 

 birth. The uterus connects with the vagina, through which the 

 sperm is introduced into the uterus and down which the young ani- 

 mal passes at birth. The genital organs and the urinary organs 

 have the same outlets from the body. 



When an egg matures in the ovary it passes down the Fallopian 

 tubes into the uterus. If no sperm is introduced, the egg is thrown 

 off infertile. In female animals an egg matures at fairly regular 

 intervals, and this corresponds with some exactness to what is known 

 as the period of heat or oestrum. 



The period of heat varies considerably in different species and 

 in different breeds and individuals of the same species. It usually 

 occurs at intervals of about three weeks. In mares it appears on 

 the ninth day after foaling, lasting two or three days. In cows it 

 appears in from three to four weeks after calving when the cow is 

 not suckling a calf, and in from six to eight weeks if she is. The 

 heat period lasts only about one day in cows. Sows come in heat 

 three days after farrowing, and not again until a week after the 

 pigs are weaned. In sheep, except for a few breeds, such as the 

 Dorset, ewes in North America ordinarily come in heat only in the 

 fall for a number of periods of two or three days each at intervals 

 of from two to three weeks. 



Females usually accept service only during the period of heat. It 

 seems to make little difference when service is made, whether early or 

 late in heat. As a rule, not more than one service is necessary dur- 

 ing one period of heat to insure conception. After conception takes 

 place, the female does not generally come in heat again during preg- 

 nancy. 



Male cells, or spermatozoa, are carried in the seminal fluid. There 

 is only one egg, as a rule, except in such animals as hogs, rats, and 

 others, which usually have several young at a birth. There are, how- 

 ever, millions of sperm cells, but only one of them is needed or used to 

 fertilize one egg. Let us now proceed to a consideration of the first 

 step in the formation of the new life. 



THE BEGINNINGS OF LIFE. 



The life which results when two animals are mated has its con- 

 trolling elements in the nuclei of the germ cells. These nuclei con- 

 tain a substance called chromatin, represented by very minute bodies 

 called chromosomes. Chromatin and chromosomes are found in all 

 cells all over the body, and the number of chromosomes in each cell 

 is the same for each species of animal. Except for the accessory 



