Fig. 4

Formation of the Schmidt’s larva in Lineus ruber. A′–G z projections of confocal scans of embryos and larvae labeled against F-actin (gray A′-D′′′) and incorporated EdU (blue E–G) counterstained with the nuclear marker Sytox Green (yellow) at 8, 10, 12 and 14 days after oviposition (dao). A′, A′′ The gastrula has a central ventral blastopore and a wide open archenteron, with isolated cells occupying the blastocoel (arrowheads A′′). B′–B′′ After 10 days of development, the embryo shows 4 paired ectodermal discs (marked by dashed lines B′), an incipient proboscis rudiment, and the ventral ectodermal ring around the blastopore (dashed circle B′). Internally, the archenteron widens and bends backwards, forming a blind gut. There are still numerous cells in the blastocoel (arrowheads). C After 12 days, an anterior indentation forms between the two anterior imaginal discs, and the whole embryo becomes covered by a thin epidermis. D′–D′′′ The mature Schmidt’s larva shows 7 separate imaginal discs covered by a larval epidermis (arrowheads D′′): the anterior terminal proboscis imaginal disc (arrowhead D′); the four lateral imaginal discs (cephalic pair and trunk pair); the imaginal disc associated with the mouth; and the imaginal disc of the gut. At this stage of development, the larva feeds on other siblings (D′′′). E–G After gastrulation, proliferative cells are scattered and distributed in the ectoderm and cells in the blastocoel (E). With the formation of the Schmidt’s larva, proliferation concentrates mostly on the regions that form the imaginal discs (arrowheads F; imaginal discs indicated by dashed lines). However, isolated cells in the space between the imaginal discs also proliferate (arrowheads G). A′–D′′, E–G are ventral views. D′′′ Is a lateral view, with the dorsal side to the top. In all panels, anterior is to the left. ac archenteron, bg blind gut, bp blastopore, id imaginal disc, iy ingested yolk, le larval epidermis, mo mouth, pb proboscis. Scale bars (A′–B′′, D′′–G) 50 μm; (C, D′) 10 μm