a. The process of fusion of the sperm and the ovum is known as fertilization. During fertilization the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilizers an ovum.The enzymatic secretions of the acrosome help the sperm enter the cytoplasm of the ovum. This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum and a secondary polar body. Then, the haploid sperm nucleus fuses with the haploid nuclues of the ovum to form a diploid zygote.
b. The zygote formed after fertilization undergoes mitosis. Mitosis takes place in the isthmus of the ovduct or the fallopian tube. This process is known as cleavage. The cleavage results in the fomation of 2,4,8,16 daughter cells that are called blastomeres.The 8 and 16 blastomeres is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus. The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass. The trophoblast gets attached to the uterine endometrium, and the process is called implantation. This leads to pregnancy.