APPENDIX 3 Cell Lineage

Abstract

Part A: Lineage Charts
Figure 1 Embryonic cell lineage of C. elegans. All interconnecting lines between the separate panels have been drawn, so that the pages can be copied, trimmed, and pasted together to give a complete chart. Vertical axis represents time at 20°C, from 0 min at first cleavage to 800 min at hatching. Many of the observations were made on eggs that were developing at slightly different rates (due to temperature variation and the effect of prolonged illumination); these primary results were normalized, by means of certain prominent cell divisions, to the course of events in eggs kept at 20°C and viewed infrequently. Precise times of individual events should not be taken too seriously; likely error varies from ±10% at the beginning of the lineage to ±2% at 400 min. Horizontal axis represents the direction of cell division. The majority of divisions have a marked anterior-posterior bias and are shown with anterior to the left and posterior to the right, without any label. Only when this would lead to ambiguity in naming the daughters is an alternative direction indicated (l = left; r = right; d = dorsal; v = ventral); thus, the system is taxonomic, rather than fully descriptive. The natural variation seen suggests that the precise direction of cell divisions is unimportant, at least in later development. Note that the daughters of a left-right division are not necessarily bilaterally symmetrical: For example, all of the cells derived from ABalaaapa lie on the left of the animal...