THE CHONDROCRANIUM OF A 20 MM. HUMAN 



EMBYRO 



JOHN D. KERNAN, JR. 



From the Anatomical Laboratory of Columbia University 



NINE PLATES 



The human chondrocrania of early stages previously described 

 have been those of 17, 18.5 and 23 mm. embryos by Van Noor- 

 den ('87), of a 30 mm. embyro by Jacoby ('94), of 13, 14, 17, 

 and 28 mm. embryos by Levi ('00), and of a 40 mm. embyro by 

 Mackhn ('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 ('00), the chondrocranium comprises four 

 primary regions, named from behind forward, the regio occip- 

 italis, regio otica, regio orbito-temporalis, and the regio ethmoi- 

 dalis (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 orbito- 

 temporal 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 at the corpus sphenoi- 

 dale. The proximal section is called the planum basale, the dis- 

 tal 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 

 to the basicranial axis. 



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JOURNAL OF MORPHOLOGY, VOL. 27, NO. 3 



