This image shows and describes the morula, blastocyst, and implantation. The text in this image follows:

• The morula, or 3-day-old embryo, is still traveling down the uterine tube. (notice the zona pellucida.)
• As a cavity forms, the rapidly changing embryo is first called a blastocyst.
• By the time implantation begins, the larger, more complex embryo is called a blastocyst.