The Growth and Development of a Viviparous Compound Ascidian, Hypsistozoa fasmeriana

Abstract

ABSTRACT The viviparity recorded for this ascidian parallels that of placental mammals in reduction of the number and size of the eggs, the development of extra-embryonic membranes, and the prolongation of larval development with increase in the gestation period. The ovum is alecithal, 25 μ. in diameter; it develops within an oviducal brood pouch in which the lining cells become distinctly modified and which remains attached to the parent for 5! months. Exposure of the embryo to the lumen of the pouch is brought about by rupture of the overlying follicle cells due to enzyme secretion by the dorsal-most cells of the blastula. Endodermal development is precocious. At the gastrula stage the future oesophageal region, the future stomach, and a pair of endodermal tubes are distinguishable. The paired endodermal tubes are unique in the ascidians. They are larval structures which disappear after metamorphosis and bear no relation to the functional stigmata of the oozooid. They resemble the branchial tubes of the Appendicularia, developing as blind outgrowths which later open through to the exterior ; but in this species they open dorsally, not ventrally. They act as passive channels through which material from the pouch passes into the gut (a bolus being present from the gastrula stage onwards) and as anchoring strands for the ectotrophe. This extra-embryonic membrane formed by elevation of the dorsal ectoderm ultimately practically envelops the embryo and becomes apposed to the lining of the pouch. It is extremely simple, exhibits no vascularization, makes no actual liaison with the oviducal epithelium, and shows little cell differentiation. Extra-embryonic material reaches the embryo indirectly by diffusion across the ectotrophe (which unlike the remaining embryonic ectoderm secretes no test), and directly through the endodermal tubes. The latter method is the more important during early stages of development, the former during later stages. The production of an extremely large and complex tadpole with numerous buds at an advanced stage of organogenesis testifies to the success of this type of viviparity.