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...