Fertilization and implantation
Read the following passage carefully and answer the given questions.
During copulation semen is released by the penis into the vagina. The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube. The ovum released by the ovary is also transported to the ampullary region where fertilisation takes place. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. The zygote undergoes cleavage and forms a blastocyst, which gets implanted in the endometrium of the uterus after about 6-7 days of fertilization. Successful fertilization leads to pregnancy.
Identify the structure, which is absent in the ovum.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Perimetrium
The ovum is the cell released from the ovary at ovulation. It is surrounded by protective layers: the zona pellucida, a membrane around the oocyte, and the corona radiata, a layer of follicle cells outside it. Between the zona pellucida and the cell membrane of the oocyte lies a small perivitelline space.
(1) Zona pellucida — A structure of the ovum. It is the membrane that surrounds the oocyte. A sperm has to touch it during fertilisation.
(2) Corona radiata — A structure of the ovum. It is the layer of follicle cells around the zona pellucida.
(3) Perimetrium — Not a structure of the ovum. It is the outer thin membrane of the wall of the uterus. The wall has three layers: perimetrium, myometrium and endometrium.
(4) Perivitelline space — A structure of the ovum. It is the narrow space between the zona pellucida and the oocyte.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
The Female Reproductive System – …
