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