Question

Assertion- After implantation, finger-like projections appear on the trophoblast and chorionic villi.
Reason- Chorionic villi are surrounded by the uterine tissue and maternal blood.
A. Both Assertion and Reason are correct and Reason is the correct explanation for assertion.
B. Both Assertion and Reason are correct and Reason is not the correct explanation for assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Hint: Chorionic villi are villi that come out from the chorion to provide maximal contact area with maternal blood.
They are an essential element in pregnancy.
Blood circulates through the capillaries of the villi, and returns to the embryo through the umbilical vein.
Thus, villi act as a border between maternal and fetal blood during pregnancy.

Complete answer:
During coitus, semen is released by the penis into the vagina.
The motile sperms swim rapidly and finally reach the ampullary-isthmus junction of the fallopian tube.
The ovum released by the ovary is also transported to the ampullary region.
Thus, fertilization takes place.
Fertilisation is the process of fusion of sperm with an ovum and thus zygote is formed.
The mitotic division starts and forms 2, 4,8,16 daughter cells called blastomeres.
Morula is the embryo with 8 to 16 blastomeres.
The blastomeres are arranged into an outer layer called trophoblast
Inner groups of cells called inner cell mass are attached to the trophoblast.
The trophoblast layer then gets attached to endometrium and the inner cell mass gets differentiated into embryos.
As a result, Implantation takes place which means the blastocyst becomes embedded in the endometrium of the uterus and leads to pregnancy.
After implantation, finger-like projections appear on the trophoblast and chorionic villi which are surrounded by the uterine tissue and maternal blood.
The chorionic villi and uterine tissue become interconnected with each other and jointly form a structural and functional unit between developing embryo (fetus) and maternal body called placenta.

So, the correct option is option B. Both Assertion and Reason are correct and Reason is not the correct explanation for assertion.

Note:
The placenta facilitated the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and waste materials produced by the embryo.
The placenta is connected to the embryo through the umbilical cord which helps in the transport of substances to and from the embryo.