Cellular interactions in the development of the shell gland of the gastropod, Ilyanassa

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Abstract
To determine the cell lineage of the shell gland in Ilyanassa, the 2d and 2c micromeres were marked with carbon granules. The 2d micromere derivatives form the shell gland, but 2c derivatives are later incorporated into the mantle. Deletion of 2d causes shell abnormalities but, in about 10% of the cases, the shell appears normal although somewhat small. Deletion of 2c also causes characteristic shell abnormalities. After deletion of both 2d and 2c, no shell forms. When 2c is marked and 2d deleted, the shell gland is marked. Any one‐quarter embryo can form internal shell masses. In 1013 isolated ectoblasts (−3A, −3B, −3C, −3D), no shell material formed. Ectoblast with any single macromere formed shell with good morphogenesis, as did ectoblast plus 4d. Posterior ectoblast plus 4d, equivalent in size to isolated whole ectoblast, formed external shell. Properly oriented ectoblast, grown in contact with a polar lobe, formed shell in up to 70% of the cases; in one‐half of these, the shell was external. Deletion of the macromere from an ectoblast plus 3C series of embryos at intervals through the first day resulted in embryos which did not form shell. Ectoblast in contact with polarlobe for six days, then separated, developed shell on the seventh or eighth day. (1) Neither ectoderm nor endoderm in isolation will form shell but any ectoderm‐endoderm combination has the histogenetic ability to form shell material; (2) the endoderm exerts its influence between the second and sixth days of development; (3) only those combinations including or in contat with polar lobe cytoplasm through the third quartet stage undergo normal morphogenesis; (4) transplanted polar lobes can induce ectoderm to form shell; (5) the histogenetic ability to form shell is suppressed or unactivated when polar lobe cytoplasm is present, except in the D quadrant; (6) specialization of the D quadrant is one of the major evolutionary trends in spiralians.