Describe the events from fertilisation to implantation in humans.

1. 1 · Define in one line. Fertilisation is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote, in the ampulla of the fallopian tube.
2. 2 · Sperm entry. The sperm meets the zona pellucida; membrane changes block other sperms; acrosome secretions help the sperm enter.
3. 3 · Completion of meiosis II. The secondary oocyte forms the ovum and a second polar body; the two nuclei fuse into a zygote. An X or Y sperm decides the sex.
4. 4 · Cleavage and morula. The zygote divides by mitosis in the isthmus into 2, 4, 8 and 16 blastomeres; 8 to 16 cells form the morula.
5. 5 · Blastocyst. As it moves into the uterus, the morula becomes a blastocyst with an outer trophoblast and an inner cell mass.
6. 6 · Draw the blastocyst. [Diagram: trophoblast, inner cell mass, blastocoel]
7. 7 · Implantation. The trophoblast attaches to the endometrium, uterine cells cover the blastocyst and it becomes embedded; pregnancy begins.
8. 8 · Close in one line. So within days, the zygote becomes a blastocyst embedded in the wall of the uterus.

Scoring tip: Use the exact words zona pellucida, acrosome, morula, trophoblast and inner cell mass. Each one is a marking point.