Human Growth and DevelopmentPrenatal Development-Embryonic Period |
When does the blastocyst form? |
The morula hollows out into a fluid-filled sphere on day four or five following fertilization. It is now called a blastocyst. By the end of the first week, the blastocyst begins to implant in the uterus.
Swarms of sperm attempt to fertilize an egg at once, though typically only one sperm penetrates the ovum successfully. LifeART image copyright (c) 2008, Lippincott, Williams & Wilkins.