Stage 9 macaque embryos studied by scanning electron microscopy.

Two macaque embryos representing the beginning and the end of stage 9 were studied by scanning electron microscopy. Morphometric evaluation shows that differential growth of head plate elevation, height of the caudal region, width of the head plate and width caudal to the neural groove exceeds embryo elongation, while width of the mid-region, embryonic disc area, width of the embryonic disc, embryo height at the mid-region, length of embryonic disc, and length of the primitive streak undergo relative shrinkage. The head plate becomes topographically subdivided into the three primary segments, while the neural groove narrows, the number of somite pairs increases from one to three, and Hensen's node is obscured. Comparison with relevant rodent studies demonstrates some topographical differences and some cytological similarities.