206 GENERAL PRINCIPLES OF BREEDING. 



in the yolk of the ovum, until it has attached itself to the walls 

 of the uterus by its umbilical system of vessels and placenta. 



4thly. The difference between the ovum of mammalia, and that 

 of birds and reptiles, chiefly consists in the larger size of the yolk 

 in the latter, which is necessary, because it has to support the 

 growth of the embryo until able to digest food by means of 

 its stomach ; whilst, in the mammalia, the placental attachment 

 affords a supply of nourishment by means of the blood of the 

 mother during the interval between the entrance of the ovum 

 into the uterus and its birth a period which embraces nearly 

 the whole time between conception and birth, and is called utero- 

 gestation. 



5thly. The period of ' heat ' is the time when one or more ova 

 are being shed from the ovarium, and are passing on into the 

 uterus ; the latter part of this period is, therefore, the best time 

 for the union of the sexes, because the semen then at once arrives 

 at the ovum, in or near the uterus, without either waiting for its 

 descent, or having to go in search of it by means of the automatic 

 power of the spei^matozoa. 



6thly. The semen will continue to retain its fructifying powers 

 for a considerable time, probably, in some cases, for many days 

 when in contact with the lining membrane of the uterus, but soon 

 ceases to be productive if confined in a vessel whose walls are com- 

 posed of dead matter. A single perfect act of impregnation in the 

 dog is, therefore, more likely to be efficacious than if followed by 

 a second, since the semen is not soon secreted again of an activity 

 equal to the first. 



