Identification of point scores at stage 23 in the rat according to the system of scoring in the human embryo.

The 8-point scores, evidenced in the human embryo of stage 23, were analysed in the rat at the end of the embryonic period. Eleven OFA rat embryos-crown-rump length 16 mm, 16th postcoital day--were submitted to serial histological sections with graphic reconstruction. The 8 internal key structures, previously observed in man by O'Rahilly--i.e. the cornea, optic nerve, cochlear duct, adenohypophysis, vomeronasal organ, submandibular gland, metanephros and humerus--were readily recognizable. The identification of these anatomical features permits to determine the end of the embryonic period and to obtain a clear distinction between this and the fetal period. The similarity and the presence of the same organs in man and rats at this period of development permits to consider the rat as a good experimental model of teratology.