Bovine Notochord Origin and Development*

Serial sections of 60 bovine embryos (23 timed specimens) from 17 through 34 days gestation were examined to determine the processes and sequence of notochord development. Head notochord formed during days 18 through 21 as the ventral‐most layer of cells from Hensen's node became intercalated as the notochordal plate between the endodermal cells of the primitive open gut. The notochordal plate responded to contact by the neural plate during day 20 with hypertrophy of notochordal cells and a ventral curving of the notochordal plate margins to form the notochordal ridge which appeared as a dorsal evagination of the gut, but was not endodermal. Formation of the head notochord into a rod (without a notochordal canal or pit) occurred during days 20 and 21 reuniting the endodermal layer ventral to the notochord. Body notochord formed during days 21 through 32–33 caudad from the level of somite pair 6 by accumulation of notochordal cells from Hensen's node directly into a rod surrounded by a basal lamina without contact or penetration of the gut endoderm. Bovine notochord was, thus, formed by two methods. After formation by both methods, the notochord enlarged by proliferation of its own cells. Dorso‐ventral undulations of the notochord became distinct by the 27th day. The processes involved in bovine notochord development were different from descriptions published concerning other species in that 1. two methods of formation were found, and 2. notochordal plate formation preceded notochordal ridge production rather than vice versa.