Fertilization

Fertilization takes place in the fallopian tubes (oviducts). During the following week the fertilized eggs (zygotes) migrate down the two tubes to the uterus.

The dog's womb (uterus) consists of two pencil-thin tubes or "horns." Under the influence of minute amounts of body chemicals called cytokines, the zygotes become evenly spaced along each uterine horn.

They implant into the wail of the uterus, then develop into two parts: embryos, which become fetuses, and placentas, which draw nutrients from the mother's bloodstream for the fetuses.

The picture on the left shows the two horns of a pregnant female's uterus and the positioning of fetuses inside the uterine horns.