MORPHOGENIC MOVEMENT AND EXPERIMENTALLY INDUCED DECREASE IN NUMBER OF EMBRYONIC SEGMENTS IN THE JAPANESE HORSESHOE CRAB,TACHYPLEUS TRIDENTATUS

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Abstract
Active morphogenic movement in the horseshoe crab, Tachypleus tridentatus, was observed during the 2 days following completion of the germ disc. Main migration of cells is as follows: Peripheral cells of the germ disc migrate to the anterior margin of the germ disc, and the cells of the anterior margin of the germ disc migrate posteriorly along the median body axis. During such migrations, the transverse furrow appears at the anterior margin of the germ disc. The 3rd prosomal segment is formed from the region just behind the furrow. By treatment of embryos with chemical reagents such as cytochalasin B, dithiothreitol and α-lipoic acid, morphogenic movement was disturbed and segment-defective embryos were formed. Defect or fusion of appendages and segments in the segment-defective embryos occurred mainly in the 3rd prosomal segment. A possible explanation is that the prospective region forming the 3rd prosomal segment cannot take part in the formation of segments as a result of the inhibition of morphogenic movement by chemical reagents.

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