A Human Embryo of Streeter Age Group XXIII

Abstract

A human embryo considered to belong to stage XXIII of Streeter's developmental horizon is described. The embryo was obtained from a total hysterectomy specimen and prepared into serial sagittal sections in order to evaluate its gestational age. The embryo was assigned to a total developmental score of 59.5 points based on numerical evaluation of the key organs, the points falling within the range of 48 to 60.5 points of Streeter stage XXIII. The crown-rump (CR) length of the embryo measures 30.5 mm and the ovulation age is estimated to be 47 ± 1 days by Streeter's criteria, and 56 days by Moore's criteria. The distinguishing features of the embryo include the brain at the five-vesicle stage with well-demarcated flexures, dense matting of retinal pigment granules, the heart with distinct four chambers and a complete partitioning of the atrioventricular canal, prominent progressive dichotomous branchings of the secondary bronchi, the rotation of the duodenum to the right, fusion of the pancreatic buds fused, the superior and inferior parathyroid glands lying on the dorsal surface of the thyroid gland, and a discrete thymus overlapping the heart. All of the key organs are marked by characteristics registering stage XXIII except for the cochlea, cartilage and bone, both of which belong to stage XXII.