The embryonic value of the micromeres inIlyanassa obsoleta, as determined by deletion experiments. III. The third quartet cells and the mesentoblast cell, 4d

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Abstract
Each of the third quartet micromeres and the mesentoblast of the fourth quartet was removed and the effects on larval development analyzed. Removal of 3a resulted in a reduction in size of the left velar lobe. Removal of 3b resulted in a moderate reduction in size of the right velar lobe. Removal of 3c resulted in the absence of the right half of the foot and usually the right statocyst. Removal of 3d resulted in the absence of the left half of the foot and the left statocyst. Removal of both 3c and 3d resulted in the absence of the foot in most cases. Removal of the mesentoblast, 4d, resulted in the absence of the intestine, heart and larval kidney and various deficiencies of the midgut. On the basis of deletion experiments, each third quartet micromere and the mesentoblast is judged to have a specific embryonic value. The generally good development of the main ectodermal derivatives of the body following removal of the mesentoblast do not suggest any role for it or its derivatives as the primary organizer of the body axis and form.