Answer
Labels for figure of pre-embryonic human development:
a. Ovulation
b. Fertilization
c. Cleavage
d. Morula
e. Early blastocyst
f. Implantation
Work Step by Step
Ovulation is the process of the release of a secondary oocyte from the ovary. If a sperm meets and fertilizes this oocyte, then it becomes a fertilized egg or zygote. Fertilization usually occurs in the fallopian tube and it is the union of the sperm nucleus and the nucleus of the secondary oocyte. The resultant structure called the zygote gets all its cytoplasm from the secondary oocyte. Therefore, male and female contributions to the offspring are not equal--right from the start.
The zygote or fertilized egg goes through a number of cleavage divisions--the original zygote becomes two cells, the two become four and the four become the eight cells. During cleavage divisions the cells increase in number but the spherical ball, the morula, that is formed does not add cytoplasm and so does not increase proportionally in size. At first, the morula is a solid ball, but later it differentiates into a peripheral layer of flattened cells, and an inner mass of cells. This is the beginning of the formation of the blastocyst, the next stage of embryonic development.
As fluid seeps in between the cells of the conceptus and accumulates in the middle, the blastocyst becomes a structure with an inner cavity ( blastocoel ) filled with fluid, and inner mass of cells ( inner cell mass/embryoblast ) surrounded by a peripheral layer of flattened cells which will form the trophoblast. The trophoblast develops into the placenta; the organs of the embryo develop from the inner cell mass.
Implantation is the attachment of the blastocyst to the wall of the uterus and its penetration into the endometrium. This usually occurs in the body of the uterus 7-9 days ( outer limits 5-12days) after fertilization.
