The determination of the shell field cells during the first hour in the sixth cleavage cycle of eggs of Ilyanassa obsoleta

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Abstract
To investigate the possibility of a relation between the moment of deletion of macromeres and the morphogenetic potential of the resulting embryo, the 2D‐, the 3D‐, or the 3(A–D)‐macromeres were deleted at regular intervals during the fifth or the sixth cleavage cycle. Ectoblasts, ‐3(A–D), are capable of extensive cellular differentiation and the formation of several organs. The developmental capacity of the ectoblasts is independent of the stage of deletion.Deletions of the D‐macromere at successive stages result in a decreasing number of internal shells; a minimum is reached 40–60 min after the fifth cleavage (24‐cell stage). Simultaneously, the number of external shells increases.Deletions of the 3D‐macromere during the first 60 min of the sixth cleavage cycle (24‐cell stage) lead to highly teratomorphic larvae; rather normal‐shaped larvae, but without heart and intestine, develop following deletions after 60 min. The moment of deletion of the D‐macromere affects morphogenesis.These results lead to the following hypothesis: each quadrant may induce an internal shell, irrespective of factors derived from the polar lobe; an internal shell is a differentiated non‐evaginated shell gland. During the first hour of the sixth cleavage cycle the 3D‐macromere suppresses shell field formation in the A‐, B‐, and C‐quadrants, and induces evagination of the shell gland in the D‐quadrant resulting in the formation of an external shell. At the same time the morphogenetic potential of the embryo is enhanced.