The time, site of origin and migration pathways of neuroblasts destined for the dorsal and ventral cochlear nuclei in the mouse have been determined autoradiographically. Neurons of the dorsal cochlear nucleus arise on gestation days 10 through 13, neurons of the ventral cochlear nucleus on days 10 through 14. Neurons of both nuclei arise within the primitive ependyma of the caudal extent of the primary rhombic lip at the level of the lateral recess; neurons of the dorsal cochlear nucleus arise caudal to neurons of the ventral cochlear nucleus. Granular cells arise on days 12 through 18 within the primitive ependyma of the secondary rhombic lip at the same levels.The time of origin within the primitive ependyma of the neuronal populatlon of each brain stem nucleus in the mouse has been determined using an autoradiographic technique with tritiated thymidine and described in abstract for most nuclei (Taber, '63, '64, '65, '67) and in detail for several (Taber Pierce, '66). The present report is limited to description of the time and sites of origin and migration pathways of the oell populations of the cochlear nuclei in the mouse.The autoradiographic technique provides a powerful and accurate method for studying histogenesis of the brain. The value of this technique has been demonstrated in recent investigations by Altman ('62a,b, '63, '64, '66, '67), Altman and Das ('65, '66), Angevine ('65), Angevine and Sidman (' 61), Berry and Rogers ('65), DeLong and Sidman ('62), Fujita and Fujita ('63), Fujita, S. ('62, '63, '64, '65), Fujita, S . and Miyake ('62), Konigsmark and Sidman ('63), Langman, Gumant and Freeman ('66), Miale and Sidman ('61), Sidman ('61), Sidman et al. ('59), Smart ('61), Smart and Leblond ('61), Uzman ('60), and Weston ('63). MATERIALS AND METHODSMethods used in this study were identical with ones used in previously reported investigations (Taber, '64, '65; Taber Pierce, '66). Pregnant mice (BALB/cGn mated with SJL/J males) were given a single injection on the tenth day of gestation with thymidine-H3, 5 pc/gm body wt and were J. COMP. NEUR., 131: 27-54.then killed 1, 8, 24, 48, 72, and 96 hours later, or their offspring killed 2 or 3 months postnatally. Similar groups wem prepared for each succeeding day of gestation until full term. To obtain a group labeled on the ninth day of gestation, laparotomy was performed under ether anesthesia and 10 pc of thymidine-H3 injected into each embryo in utero using a microliter syringe and 30 gauge needle. This procedure, which produced good labeling, was undertaken because preliminary expedments showed that the fetal circulation on the ninth day of gestation is not sufficiently developed to permit labeling by the h t procedure. Two series 04 tissues were then prepaxed.The first, referred to as the adult or postnatal series (bable l ) , consists of offspring killed 2 or 3 months after normal birth. Specimens marked "T" were perfused through the heart with 10% acrolein; 4 to 6 hours later the brains were removed and dehydrated in equal parts o f met...

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By means of autoradiography thc time, site of origin and migration pathways of neuroblasts destined for three nuclei i n the hindbrain of the mouse have been determined. The cells of the nuclei griseum pontis and corporis pontobulbaris arise on gestation days 12 through 16 in the rhombic lip from the obex to the lateral reccss. Most cells migrate within 48 hours via the pontobulbar body to the base of the pons to form the nucleus griseum Fontis; some migrate only a short distance along the pathway and collectively form tho nucleus corporis pontobulbaris. Most cells of thc nucleus reticularis tegnieriti pontis (Bechtcrew) arise in the primitive basal ependymal plate of the pons on gestation days 12 and 13 and migrate ventrally into the tegmentum. Some probably arise at the rhombic lip arid migrate via the pontobulbar body to the base of the ponline flexure where they turn dorsally into the tegmentum.

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Following injections of horseradish peroxidase (HRP) in the cerebellar cortex and nuclei of the cat, the distribution of labeled cells in the raphe nuclei was mapped. The findings confirm those made previously in studies of retrograde cell degeneration following cerebellar ablations (Brodal et al., 1960a), and in addition reveal new details in the projection of the raphe nuclei onto the cerebellar cortex and nuclei. All the raphe nuclei except nucleus linearis intermedius and nucleus linearis rostralis project onto the cerebellar cortex. The nuclei raphe obscurus and pontis contribute the greatest number of afferents to the cerebellum. With the exception of lobule VI which probably is the recipient of a weak projection, all parts of the cerebellar cortex receive afferents from the raphe nuclei. The heaviest projection is to the vermis of lobules VIIA and X, and to crus II. The afferents to the cerebellar nuclei are few in number (Tables 2-6). The observations indicate that each raphe neuron probably projects to more than one terminal site in the cerebellum. The findings are discussed with reference to other efferent and afferent studies of the raphe nuclei. All these studies indicate that the raphe nuclei have widespread efferent and afferent connections, making them capable to participate in a variety of regulatory functions.