Figure 1

Schematic drawings representing the early development of Glomeris marginata from the blastoderm stage to formation and segmentation of the regio germinalis (A) Stage 0. A uniform blastoderm and a region of enhanced cell density at the posterior pole of the future embryo, the so-called cumulus, have formed. No expression of engrailed (en) (cf. to A´). (B) The regio germinalis forms anterior to the cumulus; the ocular field appears as an anterior region of enhanced cell density. en is expressed in the future mandibular and first trunk segment (T1) (cf. to B´). (C) Segmentation of the regio germinalis begins. Transversal stripes of enhanced cell density corresponding to the later mandibular, maxillary and T1 segment appear. All anterior segments including T1 are formed from the regio germinalis; segments posterior to that are patterned from the segment addition zone (SAZ). At this stage the cumulus has transformed into the SAZ as indicated by the appearance of en expression corresponding to T2 in this region (cf. to C´). en is now expressed in the antennal and the maxillary segment (cf. to C´). (D) The postmaxillary segment primordium forms and expresses en (cf. to D´); a second stripe of en-expression appears in the SAZ (D´). (E) The premandibular segment primordium forms and expresses en (cf. to E´). en appears de novo in the ocular region and posterior in the SAZ (cf. to E´). The proctodaeum forms. Abbreviations: an, antennal segment; md, mandibular segment; mx, maxillary segment; oc, ocular region; P, proctodaeum; pmd, premandibular segment; pmx, postmaxillary segment; SAZ, segment addition zone; T1, first trunk segment.