Arrange the following events in correct sequence.

A. Formation of zygote

B. Formation of blastocyst

C. Implantation

D. Formation of morula

Choose the correct answer from the options given below:

A, D, B, C

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

A. Formation of zygote

D. Formation of morula

B. Formation of blastocyst

C. Implantation

During copulation, semen is ejaculated from the penis into the vagina, a process known as insemination. The motile sperm cells rapidly swim through the cervix, enter the uterus, and ultimately reach the ampullary region of the fallopian tube. Simultaneously, the ovum released by the ovary is transported to the same ampullary region, where fertilization occurs.

Fertilization can only happen when both the egg and sperm reach the ampullary region at the same time. This is why not all instances of copulation result in fertilization and subsequent pregnancy.

Fertilization refers to the process of the fusion of a sperm cell with an egg cell, leading to the formation of a zygote.

Following fertilization, the haploid nucleus of the sperm cell combines with that of the egg cell, resulting in the formation of a diploid zygote. The zygote initiates mitotic divisions while traveling through the isthmus of the oviduct, a process known as cleavage. These divisions give rise to 2, 4, 8, and eventually 16 daughter cells called blastomeres.

The embryo with 8 to 16 blastomeres is called a morula . The morula continues to divide and transforms into blastocyst as it moves further into the uterus. Within the blastocyst, the blastomeres are organized into two distinct layers: the outer layer referred to as the trophoblast and an inner group of cells attached to the trophoblast known as the inner cell mass. The trophoblast layer becomes attached to the endometrium, while the inner cell mass differentiates into the embryo.

Following attachment, the cells of the uterine lining undergo rapid division, enveloping and embedding the blastocyst within the endometrium of the uterus. This crucial process is termed implantation, and it marks the onset of pregnancy.