Cell nucleus development. Artwork showing the development of the nucleus in a salamander (order Caudata) egg. In figures 3 and 4 nucleoli (dark spheres) are seen around the edges of the nucleus, while in figure 6 they are beginning to aggregate in the centre. Figure 5 is a detail of a lampbrush chromosome. This is a large chromosome that results from the simultaneous transcription of many genes. Page from 'Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere' (Textbook of the developmental history of humans and vertebrates, 1906), by the German zoologist and embryologist Oscar Hertwig (1849-1922).