Explanation:
The fertilized egg moves through the oviduct propelled by involuntary smooth muscle contractions and by cilia lining the oviduct. Around 30 hours after fertilization the zygote undergoes its first mitosis and cell division. Cell division continues, and by the third day the embryo leaves the oviduct and enters the uterus. At this point, the embryo is described as a morula ---a solid ball of cells. By the fifth day, the morula has developed into a blastocyst, which can be described as a hollow ball of cells. The blastocyst attaches to the endometrium by the sixth day and is fully implanted by day 10.
