Detailed fate maps (see Chapters 1 and 2) have greatly advanced our knowledge of spiralian development. The fate maps of the gastropods Ilyanassa obsoleta and Crepidula fornicata were constructed by injecting large polymers conjugated to fluorescent dyes into specific micromeres (Render 1997; Hejnol et al. 2007). The fluorescence is maintained over the period of embryogenesis and can be seen in larval tissues derived from the injected cells. More specific results, showing divergences between snail species, have been obtained using live imaging (Figure 1; Chan and Lambert 2014; Lyons et al. 2015).
In general, the first-quartet micromeres (1a–1d), supplemented by some cells from the second and third quartets, generate the head ectoderm, while the nervous system comes largely from the first- and second-quartet cells (see Figure 1A). Fate maps confirm that the mouth forms in the same location as the blastopore. The endoderm comes from macromeres A, B, C, and D. The mesoderm comes from two sources: cells from the second and third quartets contribute the larval and adult musculature (ectomesoderm), while the majority of the mesoderm—the larval kidney, heart, primordial germ cells, and retractor muscles—comes from a particularly remarkable cell, the 4d blastomere (see Figure 1C). This highly conserved spiralian blastomere is critical for establishing the placement of the mesoderm and for inducing the formation of other cell types (Lyons et al. 2012). It is the cell most directly affected by the coiling mutation mentioned in Chapter 8 of the textbook.