DEVELOPMENT OF SPATIAL ORGANIZATION IN PALLEAL BUDS OF THE COMPOUND ASCIDIAN, SYMPLEGMA REPTANS.

The continuous observation of living animals and the reconstruction of serial sections of fixed specimens have revealed 15 developmental stages and their timetable in palleal buds of the compound ascidian, Symplegma reptans. The bud primordium (stage 1) appeared on the right anterior region of the atrial epithelium of its parental bud (stage 9). It evaginated to form a vesicle along with the epidermis (stage 3) about two days later (at 18°C). Stage 4 was characterized by the formation of a test vessel through which the bud received active bloodflow. Body axes and bilateral asymmetry first became visible at this stage. Rudiments of the neural complex, pharynx, gut, and endostyle were formed directly from the inner vesicle of the bud. The neural complex placode was the first organ rudiment observed, in 3.5-day-old buds (stage 5). It changed into a tube sac whose anterior half had cilia on the luminal surface (stage 9, eight days old); the ciliated duct thus formed. In 6-day buds (stage 8), a large cell mass could be observed histologically beneath the neural complex. The cell bodies were soon arranged at the periphery of the cell mass, thus forming the dorsal ganglion. Rudiments of the pericardium, gonad, and pyloric duct appeared first as small aggregates of mesenchymal cells at stages 7, 8, and 11 (12 days old), respectively. Muscle precursors appeared in the mesenchymal space in association with the epidermis at stage 11. They differentiated into the musculus sphincter and longitudinal body musculature during stage 12, at which time, too, the stigmata perforated the pharyngeal wall. Zooids began to feed and thus attained functional maturity after about two weeks of development from buds.