THE DEVELOPMENTAL HISTORY OF AMAROECIUM CONSTELLATUM. II. ORGANOGENESIS OF THE LARVAL ACTION SYSTEM

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

1. The digestive system of Amaroecium lacks an open archenteron at the end of gastrulation. The pharynx appears as a narrow incision with a thin roof and heavy floor. An oesophageal evagination differentiates into stomach and intestine.

2. Heart and pericardium originate from the floor of the pharynx.

3. Atrium and siphons are ectodermal structures that become associated with the digestive system.

4. The nervous system consists of a sensory vesicle enclosing two sensory masses of pigment, a hypophysis lying beside two sensory ganglion, a visceral ganglion descending laterally to the neural tube which lies to the left of the notochord throughout the length of the tail.

5. The notocord is derived from chordal cells invaginated at gastrulation. Its cells become vacuolated. The notochord is confined to the tail and posteriormost region of the trunk.

6. Muscle cells differentiate from mesodermal cells of the blastoporal margins. Asymmetry of the blastopore places the cells of its right margin dorsal to the notochord, the cells of the left margin ventral to the notochord. Each band of muscle cells consists of four longitudinal rows. Cells separate from the two lateral masses of mesenchyme and move into the body space of the developing tadpole. They give rise to muscles of the mantle.