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The effects of hypothyroidism on the development of the spinal tract of V and its relation to early fetal behavior were studied in rats from day 16 through day 20 of gestation. Hypothyroidism was induced by the administration of 0.5% Propylthiouracil mixed with rat diet beginning from day seven through term. The thyroid glands of treated and untreated control fetuses of the same age groups were examined in histologic sections. The position, size, and caudal extent of the ophthalmic and maxillomandibular divisions of the spinal tract of V were analyzed in detail with the aid of graphic reconstructions from serial sections of representative cases of both control and experimental fetuses. The ophthalmic and maxillomandibular divisions of the spinal tract of V at 15 days in the control group of fetuses extended into the second and fourth cervical spinal segments respectively, while by 19 days, both divisions of the spinal tract of V extended as far as the fourth cervical segment. In the treated group of fetuses, the ophthalmic and maxillomandibular divisions were greatly reduced in area as well as in their caudal extent into upper cervical levels compared to control groups at corresponding stages of development. At the behavioral level, the frequency of individual movements of the head, forelimbs and mouth in the treated group of fetuses was significantly reduced compared with control fetuses of the same age. Combination types of movements of head, mouth and forelimbs were severely affected both quantitatively and qualitatively in the treated group from day 18 of gestation age. It could be demonstrated that the caudal extent of the spinal tract of V corresponded very closely to the behavior repertoire seen in the fetuses of that age. We propose (1) that the spinal tract of V, in its relation with the upper cervical spinal cord levels which contain motor neurons of the spinal accessory nucleus and motor neurons which innervate the dorsal neck muscles, could play a decisive role in the integration of head and shoulder movements in early stages of development, and (2) that thyroid hormone may play a crucial role in the normal development of the spinal tract of V which is manifest in its caudal growth into upper cervical levels of the spinal cord.