The various stages of the zygote before implantation are:

In the stage ‘a’ as shown in the diagram, a zygote is formed after the fusion of male and female gametes. In stage ‘b’ as shown in the diagram, zygote enters the isthmus.

CLEAVAGE STAGE: Within the isthmus, the zygote formed by the fusion of the male and female gamete, enters the two-cell stage known as cleavage.

BLASTOMERE: The cleavage formed, undergoes mitosis. At first, to form a 2 then 4, then 8 and finally 16 cells stage. These daughter cells are called as blastomere.

MORULA: The 8-16 cell blastomere are known as morula. This morula divides continuously to form a blastocyst and moves into the uterus.

The blastomere present within the blastocyst are arranged into a layer known as trophoblast. There are cluster of interior cells attached to the trophoblast known as interior cell mass.

(5) The trophoblast layer gets attached to the endometrium lining of the uterus, the inner cells than forms an embryo after differentiation.

(6) After the attachment the uterine cells divide to cover the blastocyst.

(7) This blastocyst then, gets embedded in the endometrium of the uterus known as implantation.