2.2a	2.2b

Figure 2.2a, b Time 1 hour

The zygote (fertilised egg) has begun its development (embryogenesis). The pronuclei of the egg and spermatozoa have fused to form a diploid nucleus. Embryogenesis by progressing mitotic cell division has begun. By 1 hour the egg has cleaved once (2-cell stage) or twice (4-cell stage) - (Fig. 2.2a, b). The 4-cell stage is perhaps the easiest to distinguish between fertilised and unfertilised eggs (Fig. 2.2b). One hour after spawning an unfertilised egg either has not divided at all or has divided asymmetrically. In either case a population of eggs at this time can rapidly be scored for percentage fertilisation.