The chondrocranium of a 20 MM. Human embryo

4TESThe human chondrocrania of early stages previously described have been those of 17, 18.5 and 23 mm.embryos by Van Noorden ('87), of a 30 mni. embyro by Jacoby ('94), of 13, 14, 17, and 28 mm.embryos by Levi ('OO), and of a 40 mm.embyro by Macklin ('14).As in embyros of these early periods changes by growth are exceedingly rapid, it will be seen there are intervals to be filled.The chondrocranium here described is that of a 20 mm.human embyro, Columbia collection, no.325, reconstructed after the method of Born, at a magnification of 75 diameters.According to Gaupp ('OO), the chondrocranium comprises four primary regions, named from behind forward, the regio occipitalis, regio otica, regio orbito-temporalis, and the regio ethmoidalis (fig. 1).The structures comprising these are grouped about a central bar of cartilage to which each is joined.This scaffold is divided into two parts, chordal and praechodral (Kolliker, '49), because of the fact that the notochord extends into the first and not into the second.The pars chordalis includes the occipital and otic regions; the pars praechordalis, the orbitotemporal and ethmoid.The two divisions are not in the same horizontal plane but form (fig. 2) an angle in the orbito-temporal region of 65", the apex of the angle being a t the corpus sphenoidale.The proximal section is called the planum basale, the distal forms the interorbital septum and the mesethmoid (fig. 1).The planum basale forms the main part of the floor of the skull.It extends from the foramen magnum caudally' to the 1 Terms of direction are used here and throughout this article with reference t,o the basicranial axis.