Q.The number of nuclei present in a zygote is ……

Fertilisation is the fusion of a haploid sperm with a haploid secondary oocyte at the ampullary-isthmic junction of the fallopian tube, forming a diploid zygote. Sperm first undergo capacitation in the female tract, then penetrate the egg's corona radiata and, via the acrosomal reaction (releasing hyaluronidase), the zona pellucida; the first sperm to fuse with the egg's plasma membrane triggers cortical granules to build a fertilisation membrane that blocks any further sperm — preventing polyspermy. The zygote cleaves mitotically (blastomeres: 2, 4, 8 cells and so on) into a solid morula by about 72 hours, then into a fluid-filled blastocyst (an outer trophoblast layer and an inner cell mass) as progesterone-relaxed fallopian-tube muscle carries it toward the uterus over 4-5 days. The inner cell mass embeds in the endometrium — implantation — establishing pregnancy; implantation elsewhere (most often in the fallopian tube) is an ectopic pregnancy, a medical emergency that can cause internal bleeding or tubal rupture. Twins may be monozygotic (one fertilised egg splitting during first cleavage, identical), dizygotic (two eggs fertilised by two sperm, fraternal), or, rarely, conjoined (Siamese) twins.