THE DEVELOPMENTAL HISTORY OF AMAROECIUM CONSTELLATUM. III. METAMORPHOSIS

¹ Marine Biological Laboratory, Woods Hole, Mass., and the Biology Department, Seton Hill College, Greensburg, Pennsylvania

1. Metamorphosis in Amaroecium commences with the explosive release of secretion by the adhesive papillae and the steady, but rapid withdrawal of the caudal contents into a compact identifiable mass at the posterior end of the tadpole's trunk.

2. Reorientation of axes is initiated by strong contractions of the larval body which mark the beginning of reorganization in the internal organs.

3. The digestive loop, endostyle, yolkmass, and heart rotate through an arc of about 90° to their adult alignment by the action of the contractile force.

4. The pharynx expands and increases in depth being accompanied in its extension by the atrial or peripharyngeal cavities, the four rows of gill slits coming prominently into view.

5. Epicardial tube is differentiated directly from the yolk mass.

6. Typical thoracic, abdominal and post-abdominal regions of the tunicate body are developed by the end of the first twelve hours.

7. Ganglion and subneural glands are transformed into the adult nervous system while the products of disintegrating sensory pigment are scattered through the body.

8. Cytolysis of the caudal tissues begins after the adult organization has completely replaced the larval organization.

9. Reorientation of axes is accomplished within an hour after fixation of the larva; metamorphosis may be consideredas completed within forty-eight hours.

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