End of embryonic somite period in the rat and a comparison with the corresponding embryonic period in humans]

The end of the embryonic somite period--stage 12 according of the Carnegie Institution--was identified in the rat. The series was composed of 23 embryos taken out from uterus on day 11/4H, day 11/5H and day 11/10H post fertilization. The specimens were submitted to external macroscopic examinations and to histological sections. The rat development of this stage was characterized by the closure of the caudal neuropore, the evidence of 3 pharyngeal archs, the closure of the otic vesicle, the presence of the fore limb buds, the beginning of the cardiac septa. This stage, being similar to this of human embryos, the authors concluded that rat could be an excellent comparative model for experimental Teratology.