The embryonic value of the micromeres inIlyanassa obsoleta, as determined by deletion experiments. II. The second quartet cells

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Abstract
Each of the second quartet micromeres of Ilyanassa obsoleta was removed and the effects on larval development analyzed. Structures most often affected by removal of 2a were the left velar lobe, the left eye and the left statolith. Removal of 2b resulted in no consistent pattern of defects. Removal of 2c resulted in atypical shell development, absence of the heart, and eversión of the stomodeum; additional effects noted in some individuals involved the right velar lobe, the right statolith and perhaps the right eye. Anomalous birefringent bodies appeared frequently in the anterior region of the larva, on the right side after removal of 2c, and on the left side after removal of 2a. After removal of 2d the external shell was usually absent or rudimentary, the stomodeum was often everted, and other effects were noted in some individuals. On the basis of the deletion experiments, each second quartet micromere is judged to have a different embryonic value.