Final answer:
The correct sequence of events when a sperm encounters an egg is: the sperm reaches the ovum, a zygote is formed, implantation occurs, and then the embryo starts secreting hCG.
Step-by-step explanation:
The correct order of the process that occurs when a sperm encounters an egg is: sperm reaches ovum, zygote is formed, implantation occurs, embryo begins secreting hCG. This is outlined by several key stages in the germinal stage of development, these are fertilization, cleavage, blastulation, and implantation.
- Fertilization is when the sperm and egg unite inside the Fallopian tube, leading to the formation of the zygote.
- The zygote then undergoes a series of mitotic divisions known as cleavage, and after several days, it becomes a blastocyst.
- The blastocyst moves into the uterus and performs implantation into the uterine wall, beginning the actual pregnancy.
- Following implantation, the chorion layer of the blastocyst starts to secrete the hormone human beta chorionic gonadotropin (hCG), signaling the maintenance of pregnancy.
Therefore, the second option is the correct sequence: 'sperm reaches ovum, zygote is formed, implantation occurs, embryo begins secreting hCG'.