FERTILISATION AND IMPLANTATION ▸The process of fusion of a sperm with an ovum to form a diploid cell is called fertilisation. ▸ Fertilisation activates the secondary oocyte cell to complete the division. ▸ It normally occurs when sperm and egg interact in the upper part of the oviduct (ampulla). ▸ During fertilisation, a sperm comes in contact with ovum (zona pellucida layer) and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. ► The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the secondary oocyte ► The second meiotic division is also unequal and results in the formation of a second polar body and haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary junction. This is the reason why not all copulations lead to fertilisation and pregnancy. Cleavage is the mitotic division of the zygote unit moving through the isthmus of the oviduct towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres. ▸ Implantation occurs generally between 6th to 9th day after fertilisation. The site of implantation determines the portion of placenta.