A Human Embryo of Streeter Age Group XVII

Abstract

A human embryo considered to belong to stage XVII of Streeter's developmental horizon was described. The embryo was surgically removed from a tubal pregnancy. Although a certain portion of the back side of the embryo was lost during preparation, a total of 456 serial sections were made through the frontal plane. The ovulation age was thought to be 42 ±1 days, and the size of the embryo was llmm in crown-rump length. Characteristic features of the embryo included digital rays in the hand plate, foot plate of the lower limb bud, inward migration of retinal nuclei, definite pigment granules in the outer layer of the retina, actively elongating cochlear duct, initiation of semilunar valves, branches of the second order from stem bronchi, vermiform appendix, follicular lumen of the duodenum, fused dorsal and ventral pancreas, indifferent gonad and developing suprarenal gland. Although the embryo derived from a surgical specimen of a tubal pregnancy, there was no change that could be considered abnormal in nature. Thus the embryo was designated as a normal one.