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Figure 2 | Frontiers in Zoology

Figure 2

From: Cell lineage analysis of the mandibular segment of the amphipod Orchestia cavimana reveals that the crustacean paragnaths are sternal outgrowths and not limbs

Figure 2

(A-C) Ventral view of living embryos of Orchestia cavimana. Anterior is up. (A) A typical germ disc stage after closing blastopore with around 250 cells. The dotted line shows the multi-layered region after gastrulation. (B) The embryo in A slightly more developed. The germ disc has now about 400 cells and a dorsal organ (do) is differentiated. In the centre of the germ disc the first formation of transverse rows is visible (black arrows). (C) The embryo in A further developed. The anterior lying dorsal organ is out of focus. The first transverse rows, beginning with E(1), are arranged in the typical grid like pattern. The dotted line indicates the border between the naupliar (posterior end marked by the genealogical unit E(0)) and the post-naupliar region (anterior end indicated by genealogical unit E(1)). (D) Schematic cell division pattern of post-naupliar ectoderm rows posterior to E(1) of Orchestia as is similarly found in other malacostracan crustaceans. Top: Beginning with the segment of the first maxilla, a non-teloblastic arranged genealogical unit (row abcd) undergoes two mitotic cleavage waves in a longitudinal direction. Middle: The result is a grid like pattern of four rows (from anterior to posterior: a, b, c and d). Bottom: After the first differential cleavages (bars mark sister cells) the first morphogenesis takes place. Interestingly the later morphological segments do not conform to this genealogical unit. The segmental border (if) runs between the descendants of row b and does not match the genealogical border (gb). Accordingly, limb buds, ganglia, and other segmental organs are composed structures formed by the descendants of two adjacent ectoderm rows. The dotted lines indicate the area of the later limb buds. The anteriormost post-naupliar ectoderm row E(1) shows a somewhat different pattern (see Fig. 4 and Ref. [36]). (E) SEM-picture of an early limb bud (surrounded by dotted line) of the left second thoracopod (first pereiopod). According to the color code in 1D the red arrowheads mark the intersegmental furrows and the green arrowhead marks the border of two consecutive genealogical segments which together form a morphological segment. Medial to the limb bud (lb) the anlage of the ganglion (g) of this hemi-segment and the slightly sunken midline (ml) is visible.

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