(i) The process of a fusion of sperm with an ovum is called fertilization:
1. The motile sperms move through the cervix, enter the uterus and reach the junction of the isthmus and ampulla of the follopian tube.

2. The ovum released from the ovary also reaches the ampullary isthmic junction where fertilization takes place.

3. Fertilization can only occur if the ovum and sperms are transported simultaneously to this junction. This explains why all copulations do not lead to fertilisation and pregnancy.

4. The sperm comes in contact with the zona pellucida of the ovum and releases sperm lysins which induces changes in the egg membrane. It blocks the entry of the other sperms into the egg. Thus, it ensures that only one sperm can fertilize an ovum.

5. The acrosomal secretions help the sperm to enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.

6. This induces the completion of meiotic division of the secondary oocyte. The secondary meiotic division results in the formation at a secondary polar body and a haploid ootid (ovum).

7. The haploid nucleus of the sperm and that of ovum fuse together to form diploid zygote.
Implanation: The process of the attachment of the blastocyst to the uterine wall is known as implantation. Though, embryo reaches uterus by the end of 4th day after fertilization, it remains free in the uterine cavity for about 2 days and obtains nutrients from uterine milk. About 8 days after fertilization, blastocyst sinks into the pit formed in the uterine endometrium and gets completely buried.

The lytic enzymes secreted by syncytotrophoblast erode endometrium and enable blastocyst to penetrate uterine wail. The syncytotrophoblast is pushed into finger-like chorionic villi, which interdigitate to establish an intimate contact between blastocyst and endometrium. But the villi from exposed surface of blastocyst are lost. The association of embryo and endometrium is known as implantation. Implantation leads to pregnancy.

(ii) Implantation
The process of the attachment of the blastocyst to the uterine wall is known as implantation. Though, embryo reaches uterus by the end of 4th day after fertilization, it remains free in the uterine cavity for about 2 days and obtains nutrients from uterine milk. About 8 days after fertilization, blastocyst sinks into the pit formed in the uterine endometrium and gets completely buried.

The lytic enzymes secreted by syncytotrophoblast erode endometrium and enable blastocyst to penetrate uterine wail. The syncytiotrophoblast is pushed into finger-like chorionic villi, which interdigitate to establish an intimate contact between blastocyst and endometrium.

But the villi from exposed surface of blastocyst are lost. The association of embryo and endometrium is known as implantation. Implantation leads to pregnancy.

Fotmotlon of Embryonic Disc:
The inner cell mass of blastocyst contamin stem cells, gives rise to all the tissues and organs of the embryo.  Immediately after implantation, the cells of inner cell mass get organised into two layers, the epiblast and hypoblast. Both together form a flat embryonic disc.

Hypoblast is primitive endoderm and is formed of a layer of large calumnar cells. Epiblast is the presumptive ectoderm and is formed of two layers of cuboidal cells. A small cavity appears between the two layers of epiblast which develops intc amniotic cavity.