The 4d blastomere not only develops autonomously, but also induces other cell lineages. The Notch signaling pathways may be critical for these inductive events of the 4d blastomere. Blocking Notch signaling after the 4d blastomere has formed causes the larva to resemble those formed when the 4d cell is removed (lacking heart and gut), whereas the other autonomous fates of the 4d cell (such as larval kidneys) are not disturbed (Gharbiah et al. 2014). The D set of blastomeres is thus the “organizer” of snail embryos. Experiments have demonstrated that the nondiffusible polar lobe (cortical) cytoplasm that is localized to the D blastomere is extremely important in normal molluscan development for several reasons:
It contains the determinants for the proper cleavage rhythm and the cleavage orientation of the D blastomere.
It contains certain determinants (those entering the 4d blastomere and hence leading to the mesentoblasts) for autonomous mesodermal and intestinal differentiation.
It is responsible for permitting the inductive interactions (through the material entering the 3D blastomere) leading to the formation of the shell gland and eye.
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