Structural and morphological changes were examined in the inferior olivary complex of 25 rhesus monkeys that were 60 days gestation to three months after parturition. At different ages, onehalf brain stem was sectioned sagittaly, and when possible the symmetrical half was sectioned in either the coronal or the horizontal plane. Serial sections were stained by the silver reduction method of Stotler ('51).The olivary complex undergoes its major development between 60 and 129 days gestation when its length, cellular morphology, and afferent patterns resemble those of the adult organ. Structurally, the three m d o r divisionsthe principal nucleus, the dorsal accessory nucleus, and the medial accessory nucleus-can be identified at 60 days gestation. At that time, the medial accessory nucleus, the largest of the three, can be subdivided into a ventrolateral and a dorsomedial section; as it develops, it extends caudally and acquires a small cap of cells on its dorsomedial part. By 80 days, the dorsal lamella of the principal nucleus, initially smaller than the ventral, equals the ventral lamella in size and continues to enlarge until by 100 days it has developed two large sulci. The dorsal accessory nucleus, the smallest of the three divisions, initially has a distinct connection with the caudal part of the dorsal lamella of the principal nucleus but it is lost by 92 days.Major afTerents to the inferior olive could easily be identified in the early fetal tissue -before the development of the dense neuropil. M e r e n t s first arise from the lateral funiculus of the spinal cord at 60 days and project primarily onto the ventrolateral part of the medial accessory. As they develop, they gradually project more rostrally along the ventrolateral section, onto the dorsomedial section of the medial accessory, onto the caudal part of the ventral lamella, and along the caudomedial part of the dorsal accessory. At 92 days gestation the central tegmental fasciculus, most of which originates from the red nucleus projects into the rostral and later into the middle section of the dorsal lamella, into the rostral ventral lamella, and a few into the rostrolateral part of the dorsal accessory and to the rostral third of the medial accessory. A few fibers arising from the corticospinal tract enter the caudal section of medial accessory, and a few fibers project vertically from the dorsomedial tegmental region of the medulla into the medial section of the dorsal accessory.The inferior olivary complex has been mental morphological and electrophysiologthe subject of numerous morphological ical studies of afferents which project onto investigations. Descriptions have been distinct portions of the olivary complex. made of the important aspects of its emAfferents from the spinal cord have been brvolom (Essick, '12; Harkmark. '54). its -_ .
