Answer: The option with the correct order is D and it includes:
--> sperm reaches ovum,
--> zygote is formed,
--> implantation occurs
--> zona pellucida dissolves,
--> and the embryo begins secreting hCG.
Explanation:
In humans, the male ejects sperms with great force into the cervix and the upper part of the vagina of the female. The sperms move rapidly through the cervix and the uterus into the upper part of the oviduct ( the fallopian tube) where it REACHES THE OVUM.
There are follicle cells and the zona pellucida which surrounds the ovum. The acrosomal lytic enzymes in the head of the sperm can dissolve the cell wall and membranes. They help the sperm to move through the follicle cells that surrounds the ovum and penetrate the zona pellucida of the ovum. The sperm them moves into the ovum. The nucleus of the sperm fuses with that of the ovum to FORM A DIPLOID ZYGOTE.
The zona pellucida undergoes some changes that prevents other sperm from penetrating the ovum. It continues to protect the zygote as it travels through the fallopian tube to the uterus, the site for implantation. Around 72 hours after fertilization, IMPLANTATION OCCURS and the ZONA PELLUCIDA FRAGMENTS AND DISSOLVES.
Soon after implantation, the embryo begins secreting Human chorionic gonadotrophin (HCG) through the trophoblast cells which surround the developing embryo. It is the embryonic hormone that ensures the corpus luteum continues to produce progesterone throughout the first trimester of pregnancy.
