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Figure 2   Development of pluteus with skeletal defect from a "simultaneous three." (a). Diagram of the trefoil with cells A, B, and C. (b). Pluteus in same orientation as (a). Each sector has uniform nuclei, shown only in the anal region. Note especially small size of nuclei in sector B and absence of skeleton in sector A (m, mouth; s, apical rod; a, anal rod). From Boveri (2).

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