Ans. is 'c' i.e., 16 cells Feilization and implantation Feilization refers to fusion of male and female gametes (i.e. spermatozoon and ovum). It takes place in the middle segment (ampulla) of fallopian tube. Before feilization, the ovum and sperms reach the ampulla for feilization. Fusion of spermatocyte and ovum leads to formation of zygote. First week of development begins immediately after feilization and includes :? i) Cleavage of zygote : Zygote (feilized ovum) stas dividing immediately and large zygote is subdivided into smaller daughter cells called blastomeres. Blastomeres are still surrounded by zona pellucida. Cleavage occurs in fallopian tube (uterine tube). ii) Formation of morula : At about 16 cells stage the blastomeres tightly align by the process of compaction to form a compact ball of cells called morula (mulberry). This process of compaction leads to segregation of cells into two groups (i) inner cells (inner cell mass), and (ii) outer cells (outer cell mass). Morula enters uterine cavity 4 days after feilization. iii) Formation of blastocyst : As the morula enters the uterine cavity, uterine fluid diffuses through zona pellucida and fills small intercellular gaps between blastomeres, and morula is conveed to blastocyst. Blastocyst consists of :? a) Zona pellucida : Outer covering. b) Embryoblast : A group of centerally located cells of inner cell mass and later give rise to tissues of embryo proper. c) Trophoblast : A thin outer layer of cells formed from outer cells mass and later give rise extraembryonic tissues. d) Blastocele : Cavity of blastocyst : The region of blastocyst containing embryoblast is known as embryonic pole and the opposite pole, the abembryonic pole. The trophoblasts overlying the embryoblast at embryonic pole is called polar trophoblast and that occupying the rest of wall called mural trophoblast. Between 5-6 days after feilization, blastocyst hatches from zona pellucida, and this naked blastocyst is ready for implantation