A Human Ernbryo of Streeter Age Group XV

Abstract

A human embryo was serially sectioned and was reconstructed with 327 sections. The length of the embryo was estimated to be 6.9 mm. The age of the embryo was determined by the developmental status of the internal organs in terms of selected definable characteristics. Discernible morphological features of the embryo included closure of the lens vesicle, separation of the otocyst from the skin ectoderm, endolymphatic appendage, bronchi in the primary stage, thickening of the olfactory disk, dorsal and ventral pancreatic buds, and a narrow atrio-ventricular canal with an incompletely formed interventricular septum. The above observations strongly suggested that this embryo developed normally and should be classed in age group XV of Streeter's developmental horizon.