Question

Which of the following is the first change that occurs to the zygote after fertilization?
A. It divides to form a hollow ball of cells, called the blastocyst.
B. It begins to secrete the hormones.
C. It contacts the endometrial wall of the uterus and becomes buried inside it.
D. It initiates the formation of placenta.

Answer 1

Hint: A zygote, also recognized as a fertilised egg, is the stage of development in which the egg and sperm combine to form a single cell. The zygote has a complete set of chromosomes, including 23 from the egg and 23 from the sperm. The zygote phase lasts only four days before the single cell splits into a blastocyst and then an embryo.

Complete Step by Step Answer:
A blastocyst is a fertilised egg's cluster of dividing cells. It's an embryo in its early stages. It is one of the various steps that eventually lead to pregnancy.
Within five to six days after a sperm fertilises an egg, a blastocyst develops. Cell layers in the blastocyst split and detach. They eventually build into structures that safeguard and nurture the growing fetus (unborn baby).
One egg is released by the ovaries 14 days after a person's menstrual period begins (ovum). The egg enters one of its fallopian tubes and awaits fertilisation by a sperm.
If fertilisation does not take place, the egg enters its womb (uterus) and exits the body during its next menstrual period. However, if fertilisation occurs, their fertilised egg develops into a zygote, a single cell with genes from both parents.
Three to five days after fertilisation, the zygote traverses through the fallopian tube to the uterus. The zygote cell divides indefinitely, eventually creating a hollow ball of cells known as the blastocyst.
The blastocyst remains in one‘s uterus for several days prior to actually implanting in the uterine wall's inner lining (endometrium). It continues to produce new cells, which form layers. The blastocyst develops into an embryo about 10 to 12 days after fertilisation. It is still an embryo until around nine weeks after implantation when it transforms into a fetus.
The correct answer is option A.

Note: Cells on the blastocyst's outer layer are attached to the endometrium's outer layer while cells in the blastocyst's internal layer implant more profoundly in the endometrium. These cells develop into the placenta, which transports oxygen and nutrients to the growing fetus.