610 Child Development In surgery ing fror 169 em (E69)to the average gestat 165 es were er similar to postr be visual zed as on the the spective princ sulcus the fetal tis sufficient tim Dat nves- These aut adiog the rth of cortico callos eric)and prisingly.pe on on opmen av ons and wide limits can be d d from analysis of stissue prepar ith con ed the mode of deve ent of the co de d his a h ma o the bir ay ergntweeo977 Ral Tho of the feus 68,1 present in the s (Fig A z the callosum fr the in alread 610 Child Development above the structure in which radioactivity was present (for examples, see Goldman & Nauta, 1977a, 1977b; Goldman-Rakic, 1981a). Development of prefrontal efferents.— In die studies mentioned, prenatal surgery was performed on timed-pregnant rhesus monkeys carrying fetuses ranging from 69 em￾bryonic days (E69) to 152 embryonic days (E152) and on several newbom monkeys. In diis species, the average gestation lasts 165 days. The fetuses were exposed dirough an incision in the uterus and were operated on in a manner similar to postnatal neurosurgeiy. Although the primary fissures do not develop fully until the last third of gestation, the prin￾cipal sulcus can be visualized as a small in￾dentation on the dorsoktera] surface of the frontal lobe; in younger fetuses, the iiyection was placed in die center of the dorsolateral part of the lobe. After microinjections of radioactive amino acids were made in the pro￾spective principal sulcus, the fetal tissues were sutured and the fetus returned to the uterus, which was also sutured. The fetuses survived 1-3 days, sufficient time for trans￾port to occur in die padiways under inves￾tigation. These autoradiographic Investigations were designed to provide basic information on timetables for the outgrowth of cortico￾cortical (callosal and intrahemispheric) and selected cortico-subcortical connections. Sur￾prisingly, perhaps, precise information on the development of connectivity was not avail￾able for any species, since only gross approxi￾mations and wide limits can be deduced from analysis of nervous tissue prepared with con￾ventional stains. Another basic question con￾cemed the mode of development of the co￾lumnar pattem of callosal innervation characteristic of the adult pathways. We wondered whether diis input was initially diffusely distributed in the cortex before be￾coming segregated into distinct colmnnar territories in a manner similar to the bii^asic development of afferent territories in the vi￾sual cortex (Hubel & Wiesel, 1977; Rakic, 1976). Thorough examin^on of the youngest fetus injected, E69, provided no evidence that callosal or intrahem^pheric axons were yet present in the vicinity of their target struc￾tures (Fig. 4). Although labeled axons could be seen entering the callosum from the in￾jected hemisphere, none reached and were present in the opposite hemisphere at this age. In contrast, labeled fibers were already evident in the caudate nucleus, which is part of the basal ganglia and also in die thalamus E69-E70 P4-PII FIG. 4.—Gallosal fibers at different embryonic (£) and postnatal (P) ages. The first number be￾neatii each drawii^ indicates the age in days when Uie animal was injected witii radioactive tracers, and the second number of tihe pair indicates the ^ e in days when it was sacrificed. Callosal fibers do not reach the contralateral hemisphere until after E70 and do not enter the cortica] plate in significwit numbers before E123. Callosaj axons invade the cortex between E123 and E152. At around E135 (not shown) and E152, they are already well seg￾regated into the modular pattem chaiacteristic of the neocortex in the postnatal monkey (from Gold￾man-Rakic, 1981b)