MORPHOLOGY OF THE OVOTESTIS OF SYNAPTULA-HYDRIFORMIS (HOLOTHUROIDEA, APODA) - AN EVOLUTIONARY MODEL OF OOGENESIS AND THE ORIGIN OF EGG POLARITY IN ECHINODERMS

The viviparous holothuroid Synaptula hydriformis, a simultaneous herma phrodite, broods its young in the coelom and releases young year round . Both eggs and sperms develop asynchronously in the two short gonadal tubules that are suspended in the coelomic cavity. Sperms are release d into the tubule, but eggs are not. Each gonadal tubule consists of p in outer peritoneum composed of flagellated epithelial cells, muscles, and nerves; an inner germinal epithelium of germinal and somatic cell s; and a middle connective-tissue (hemal) compartment bounded by the b asal laminas of the peritoneum and germinal epithelium. Spermatogonia and oogonia are flagellated epithelial cells that retain epithelial po larity throughout gametogenesis. During spermatogenesis, spermatocytes emerge towards the lumen from the germinal epithelium and sperms ente r the lumen apex (tail) first. Supportive and phagocytic interstitial cells are associated with spermatocytes. During oogenesis, oocytes sub merge basally into the hemal sinus and carry the epithelial basal lami na ahead of them. The oocyte apex, however, maintains adhering junctio ns with somatic (follicle) cells and bears a flagellum, at least into the vitellogenic phase. Full-grown oocytes are enclosed in basal lamin a, except apically, where an apical protuberance forms. The apical pro tuberance is a region of ooplasm, devoid of yolk and rich in microtubu les, that terminates apically in an extensive junction between the ool emma and follicle cells of the germinal epithelium. The oogenic patter n of S. hydriformis indicates that the apical-basal polarity of the oo cyte becomes the animal-vegetal polarity of the egg, which in turn, be comes the anterior-posterior axis of the larva. Homologous oogenic pat terns occur in crinoids and ophiuroids, but not asteroids and echinoid s. An outgroup analysis of echinoderm oogenic patterns with those of C ephalochordata and Cnidaria suggests that the pattern common to holoth uroids, crinoids, and ophiuroids is plesiomorphic to echinoderms and, in general form, to deuterostome metazoans.