Based on the principle of binocular stereopsis, three-dimensional reconstruction of the nucleus isthmus (NI) in the frog (Rana nigromaculata) has been realized by using a microcomputer system. The histological techniques used were somewhat different from conventional ones in that two pin holes, which were regarded as fiducial points to ensure an accurate alignment of the outlines of microstructures in the serial sections during three-dimensional computer reconstructing, were made in the tissue block with a micropipette before it was cut. All operations such as collection of outline-data of the microstructures, data processing and drawing output were all under control of the software. Three-dimensional morphology of NI and its location in the midbrain have been demonstrated by the stereopairs. Its dimensions measured on the basis of the reconstruction is 500 (caudorostal)?810 (dorsoventral)?810 (mediolateral)?m. The significance of three-dimensional computer reconstruction for accurately measuring the dimensions of NI and other biological microstructures and for the study of histological morphology are discussed.

According to the principle of shading, the nucleus isthmi (NI) from the frog (Rana nigromaculata) and lizard (Gekko gekko) were reconstructed in three-dimensions by using a microcomputer system. The dimensions and volume of NI in the two species were measured. A comparison of the three-dimensional structure of NI from the frog, lizard and pigeon indicates that the more advancedis is the species of lower vertebrates in evolution, the more complex is the three-dimensional structure of their NI.