The Role of the First Quartet Micromeres in the Development of the PolycladHoploplana inquilina

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Abstract
The role of the first quartet micromeres at the eight-cell stage in the development of the polyclad flatworm Hoploplana inquilina was analyzed with regard to specific contributions made by these cells and their function in the determination of embryonic symmetry. The experimental series involved: (1) deletion of one micromere (1a or 1c versus 1b or 1d); (2) deletion of two adjacent micromeres; (3) deletion of three micromeres; (4) isolation of intact first quartets; and (5) isolation of macromere sets 1A-1D. As the number of micromeres removed was increased, the larvae became progressively more abnormal, involving reduction in number of eyes, deficiencies in lobe development, and disturbance of embryonic symmetry. After deletion of three micromeres, none of the larvae exhibited normal morphology. These experiments indicate that the determination of embryonic axes leading to a larva with bilateral symmetry may involve micromere-macromere interactions, as has been shown in molluscan embryos with equal cleavage. Isolated first quartets consistently formed spherical, bloated, transparent larvae with multiple eyes, suggesting that the macromeres play an inhibitory role in eye development. Isolated macromeres 1A-1D often failed to develop, and larval structures never differentiated. Thus, the relatively loose determination of the polyclad embryo involves both cytoplasmic localization and cell interactions.