Because the major difference between cases 1 and 2 concerning the involvement of the lateral bend of the PO, the results indicate that NB projects to this olivary region. Structure of the Macaque’s Red Nucleus Olivary injection sites (Fig. 4). Case 3 comprised 6 injections of 1.0-ml, a 45-hr wait for tissue preparation and resulted in an injection site that encompassed the rostral half of the olivary complex. The tracer was restricted to the rostral parts of MAO, DAO and PO, with additional partial involvement of the surrounding reticular formation. Therefore the rostral (XII–VIII level) part of dl of PO was heavily stained, but the caudal (VII–V level) part of dl of PO was stain-free. Case 4 also comprised 6 injections of 1.0-ml and a 45-hr wait for tissue preparation. It encompassed the middle part of the olivary complex. The rostral (XII-XI level) part of dl-PO and most of the MAO were stain-free. The caudal (VIII–V) part of dl of PO and a restricted spot in the middle of MAO were heavily stained, with additional partial involvement of the surrounding reticular formation. Distribution of labeled cells in the mesodiencephalic area (Figs. 5,6,7). Labeled neurons in case 3 (Fig. 5) were observed ipsilaterally throughout the extent of the ND (Figs. 5, 7A and C), but some labeled cells were seen in the NB. Although many labeled cells were observed in the dmpNr (Figs. 5 and 7E), no New Model of Human Red Nucleus labeled cells were observed in the well-developed vlpNr. No labeled cells from case 4 (Fig. 6) were found in the caudal half of ND (Figs. 6 and 7D), whereas some labeled cells were seen ipsilaterally in the dmpNr, NB and the rostral half of ND (Figs. 6 and 7B). Many labeled neurons were observed ipsilaterally in a restricted area of the well-developed vlpNr (Figs. 6 and 7F). Owing to the major difference between cases 3 and 4 concerning the rostral part of both rostral MAO and dl of PO, the rostral and caudal parts of the ND can be said to project to the caudal and rostral parts of the rostral MAO, respectively, and the dmpNr and vlpNr project to the rostral and caudal part of the dl of PO, respectively. Fronto-Rubral Projection of the Macaque (Figs. 8 and 9) Multiple injections of 10.6-ml WGA-HRP solution into 23 sites were performed into premotor and FEF areas, but sparing F5 (see Matelli et al., 1985 [11]) and the leg-foot region of the premotor area in the macaque (Fig. 8) for case 5, and after a 41-hr wait provided before tissue preparation. In the rostral diencephalons, labeled descending fibers appeared gathered in the medial part of the crus cerebri and scattered labeled descending fibers were observed ipsilaterally in the subthalmic nucleus, the zona incerta and adjacent thalamic nuclei. Many labeled terminal arborizations were seen ipsilaterally in the dmpNr and the medial part of the vlpNr, and some labeled terminal arborizations were observed in Figure 5. Distribution of WGA-HRP-labelled cells in mesodiencephalic structures (case 3). Drawings are arranged rostrocaudally; section A is the most rostral. One dot represents one labelled cell. Large dot shows a large labelled cell. dmpNr–dorsomedial part of parvicellular red nucleus, FR–fasciculus retroflexus, mNr–magnocellular red nucleus, NB–nucleus accessorius medialis of Bechterew, ND–nucleus of Darkschewitsch, Nint– interstitial nucleus of Cajal, vlpNr–ventorlateral part of parvicellular red nucleus, III–oculomotor nucleus. doi:10.1371/journal.pone.0006623.g005 PLoS ONE | www.plosone.org 7 August 2009 | Volume 4 | Issue 8 | e6623