amphibia), with the nodal region corresponding to the dorsal lip. The primitive streak may be considered to be generally homologous with the blastopore of lower vertebrates (e.g. Thus, the streak extends half way along the disc in the stage 6 embryo, reaching its greatest relative length in stage 7 and its maximum length in stage 8.įormation of the primitive streak is induced by the underlying visceral hypoblast, which remains beneath the streak even at later stages.

The relative dimensions of the primitive streak and the fates of the cells that pass through it change with the developmental stage. With the development of the streak, the terms medial and lateral can be used. Although the future rostral (cranial) and caudal regions of the embryo are well within the boundaries of the embryonic disc, it has become the practice to term the region of the disc closest to the streak ‘caudal’, and the region of the disc furthest from the streak ‘rostral’. At this stage the primitive streak is first seen in the caudal region of the embryonic disc as a collection of pluripotent cells, orientated along its long axis in the median position and conferring the future craniocaudal axis of the embryo ( Figs 10.1– 10.2). Seen from the dorsal (epiblastic) aspect at stage 6, the embryonic disc appears elongated. Susan Standring MBE, PhD, DSc, FKC, Hon FAS, Hon FRCS, in Gray's Anatomy, 2021 Primitive streak and node

Blastopore formation