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The subcommissural organ (SCO) of the Bradypus tridactylus was studied from the histological and ultrastructural point of view. The methods of GOMORI (alum-chromic-hematoxylin-phloxine), AZAN (M. Heidenhain), KLUVER and BARRERA (for nerve cells and myelinated fibers) and silver impregnation according to PALMGREN and CAJAL-DE CASTRO, were used. The material prepared for ultrastructural analize was submitted to the sequence of gluteraldehyde, osmium tetroxide, uranyl acetate and lead citrate. In function of the secretory phenomenon, a systematic study is made of different hystological and ultrastructural characteristics of the SCO and hypendymal rosettes, which were considered in the B. tridactylus as a morphofunctional unit. Fine vascular and nerve networks were observed, by the basal parts of the compact tubule alveolar system (hypendymal rosettes) which composes part of the SCO. Nerve fibers were seen between its cells reaching even up to the lumen, and they may end there in bulbar dilatations which seem to show secretory characteristics at the ultrastructural examination.

On a study of the most rostral part of the optic tectum of Clemmys caspica leprosa a periodically striated structure was detected at the subcommissural organ (SCO). It has been made a detailed analysis about its topographical relationship with vessels and with hypendyma, and also about its own periodical pattern of structure. This finding was also considered on a comparative point of view, keeping in mind the previous observations by some authors upon the rat's SCO and other related findings outside the central nervous system. Finally a functional interpretation is discussed.