STRUCTURE OF THE AMPHIBIAN MESONEPHRIC TUBULE DURING ONTOGENY IN RANA-RIDIBUNDA L TADPOLES - EARLY ONTOGENIC STAGES, RENAL CORPUSCLE FORMATION, NECK SEGMENT AND PERITONEAL FUNNELS

The morphological changes produced in the mesonephric tubule during on togenesis, not previously reported in amphibians, are described in Ran a ridibunda tadpoles using light and electron microscopic methods. The rudimentary nephron units do not develop synchronously along the subp eritoneal nephrogenic ridged cord. The first signs of morphogenesis ar e the presence of round euchromatinic nuclei and mitotic figures. The subsequent developmental stages are characterized by detachment of the rudimentary nephrons from the nephrogenic cord. Renal corpuscle forma tion is characterized by glomerular expansion, differentiation of larg e fenestrated capillaries and the presence of a discrete mesangium and a small capsular space. Interstitial capillaries next to the renal co rpuscle rudiments appear to induce invagination and differentiation of the capsular epithelium. Developing podocytes were cuboidal undiffere ntiated epithelial cells with scarce primary processes and with an ext ensive part of the cell surface lying flat on the glomerular basement membrane. These features reflect low or no glomerular filtration durin g nephron development. The ciliated neck segment and peritoneal funnel s show similar structural features. The latter were not physically con nected with the nephrons, but opened into renal blood vessels. Involut ive peritoneal funnels were observed.