|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
|
Fig. 1. Events in the N lineage leading to gangliogenesis in H. robusta.
Cleavage yields an embryo with a posterior growth zone of five bilateral pairs
of segmentation stem cells (teloblasts); only the four pairs of ectoteloblasts
are shown here. Ignoring gastrulation, segments arise in anteroposterior
progression along the ventral midline by the stereotyped divisions of columns
of segmental founder cells (blast cell bandlets). In the N (and Q lineages),
clones of two successive blast cells, designated ns (red) and nf (blue)
contribute distinct sets of progeny to each segment. Because blast cells of
each type execute a stereotyped pattern of cell division relative to the time
of their birth from the parent teloblast, events in the generic nf and ns
lineages can be designated as occurring at a specific `clonal age' (cl.ag.),
as indicated on the time line on the left. Primary n blast cells divide
unequally, producing larger anterior (nf.a, dark blue; ns.a red) and smaller
posterior (nf.p, light blue; ns.p, pink) daughter cells (40 and 44 hours
cl.ag., respectively). Transverse fissures (arrows) separate nf.p clones
(light blue) from ns.a clones (red) dividing the bandlets into discrete
ganglionic primordia (85-95 hours cl.ag.). Anterior is upwards in all figures
unless otherwise noted.
|
