Source: Human Fertilization image, Creative Commons: http://commons.wikimedia.org/wiki/File:Human_Fertilization.png; Blastocyst image, Creative Commons: http://commons.wikimedia.org/wiki/File:Blastocyst.png
The fertilized egg which is the first cell of an individual.
After the morula forms it will implant on the endometrium of the uterus; the blastocyst contains an inner of layers of cells that become the embryo while the outer layer of cells form the placenta.
A layer of the blastocyst which will develop into an embryo.
A stage of development of an individual formed from the inner cell mass of a blastocyst after it has implanted in the uterus.