The progression from a fertilized oocyte to a newborn puppy is a remarkable phenomenon that occurs in a period of approximately two months. Embryonic development encompasses the period of time at which three germ layers differentiate: ectoderm, mesoderm, and endoderm. Organ systems are formed from these germ layers, with most of the reproductive tract being derived from mesoderm. Organogenesis is complete prior to the fetal stage in canine embryos, but sexual differentiation occurs during the fetal stage. Sexual differentiation is a well-coordinated progression of events that is directed initially by the genotype of the developing embryo and fetus. Developing fetuses are inherently female and will develop as such in the absence of a Y chromosome. Male fetuses develop as the Y chromosome causes regression of the female duct system and development of the male duct system. Testicular descent in the canine begins in the fetal stage, but is not completed until after birth.