The embryonic value of the micromeres in Ilyanassa obsoleta, as determined by deletion experiments. I. The first quartet cells

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Abstract
Each of the first quartet micromeres of Ilyanassa obsoleta has been removed at the 8‐cell stage, and the effect of the loss observed in the larva.Removal of the 1a micromere resulted in all cases in absence of the left eye; a small reduction in size of the left lobe of the velum could frequently be detected also. In other observed details the larvae appeared complete.Absence of the 1b micromere resulted in a variable tendency toward synophthalmia, and some indication of reduction in size of the velum. Sporadic abnormalities of other types were encountered in the experimental series.Loss of the 1c micromere resulted in absence of the right eye, a slight reduction in size of the right lobe of the velum and, probably, absence of the right tentacle.Removal of the 1d micromere was without observed effect on larval structure in two out of three cases.The results are compared with deductions from cell lineage work on the related form Crepidula, and they are considered also from the standpoint of regulative capacity. The Ilyanassa embryo shows both mosaic and regulative features.