Fig. 2

Continuity of forms in time. A Cytology. Cell cycle, after [51], modified by excluding centrioles and microtubules. Supercoiling of the DNA leads to chromosome condensation during the prophase and re-condensation during late anaphase and telophase. Since a nuclear membrane is considered to be constitutional for the cell nucleus and this membrane is absent from the metaphase to the early telophase, a cell nucleus is not present during these phases. 1 end of S-phase, 2 prophase, 3 pro-metaphase, 4 metaphase, 5 anaphase, 6 late anaphase, 7 telophase, 8 start of S-phase. B Embryology. Neurulation in the chordate development. Ectodermal cells form a fold by differential contraction of the apical actin filament belt. This fold becomes tube-like, detaches from the ectoderm, and forms an epithelial tube underneath the ectoderm. The tube develops into the medullary cord of the central nervous system. Blue basal lamina, green epithelial surface. C Evolution. Formation of the fore-limb of the horse (redrawn from different sources). During the course of evolution, the third toe becomes re-enforced, while toes 1, 2, 4, and 5 are successively reduced (C1 †Hyracotherium leporinum, C2 †Mesohippus biardi, C3 †Merychippus insignis, C4 Equus ferus)