Slide showing a graph demonstrating 'the part played by the activation centre and the differentiation centre in the development of a regulative insect egg.' Slide shows a graph demonstrating 'the part played by the activation centre and the differentiation centre in the development of a regulative insect egg.' The graph's 'Y' axis demonstates 'distance from the posterior end of the egg (1 unit = 24 [symbol])' starting from 0 to 37. The 'X' axis shows five eggs labelled 'a' to 'e'. Text below graph reads: 'Fig. 261 - diagram showing the part played by the activation centre and the differentiation centre in the development of a regulative insect egg. (a) activation centre not yet activated; (b) beginning of its function; (c) differentiation centre; (d) blastoderm arising; (e) completion of blastoderm. Ko, headfolds.' Late 19th to early 20th century