Arrange the following events in the correct order pertaining to ferti…

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

(D) The zygote divides mitotically and becomes a morula — first. Cleavage starts as the zygote moves down the oviduct. The embryo with 8 to 16 blastomeres is the morula.

(C) The blastomeres are arranged into trophoblast and inner cell mass — second. The morula changes into a blastocyst. Its outer layer is the trophoblast and its inner group of cells is the inner cell mass, which will form the embryo.

(A) The blastocyst becomes embedded in the endometrium — third. The trophoblast attaches to the wall of the uterus and the blastocyst sinks into it. This is implantation, and it leads to pregnancy.

(B) Chorionic villi appear on the trophoblast — fourth. These finger-like projections form only after implantation. With the uterine tissue they form the placenta.

NCERT Extract:
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). The morula continues to divide and transforms into blastocyst (Figure 2.11g) as it moves further into the uterus. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. 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.
After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood.”