Arrange the following events in the order in which they occur in huma…

The correct answer is Option (1) → (B), (D), (A), (C)

In humans, fertilisation takes place in the ampullary region of the fallopian tube. The events after it follow the path of the young embryo down the oviduct and into the uterus.

(B) Fertilisation comes first. A sperm fuses with the ovum in the ampullary region and a diploid zygote is formed.

(D) Cleavage is next. The zygote divides by mitosis as it moves through the isthmus towards the uterus, forming blastomeres, then a morula and finally a blastocyst.

(A) Implantation follows. The trophoblast of the blastocyst gets attached to the endometrium, and the blastocyst becomes embedded in the uterine wall.

(C) Formation of chorionic villi comes last. Only after implantation do these finger-like projections appear on the trophoblast; together with the uterine tissue they form the placenta.

So the correct order is B, D, A, C.

NCERT Extract:
Fertilisation and Implantation – “… The ovum released by the ovary is also transported to the ampullary region where fertilisation takes place.”
Fertilisation and Implantation – “… The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus (Figure 2.11) and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula (Figure 2.11e).”
Fertilisation and Implantation – “… The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus (Figure 2.11 step H). This is called implantation and it leads to pregnancy.”
Pregnancy and Embryonic Development – “After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood.”

Figure 2.11 Transport of ovum, fertilisation and passage of growing embryo through fallopian tube