312 CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



is shown intact after removal of the nasal capsule. The large crista galli and 

 anterior end consist largely of young cartilage and precartilage. 



INFERIOR ASPECT. 



In comparing the inferior surface of the chondrocranium with that of the adult, 

 only the occipital and otic regions lend themselves readily to our purpose, unless we 

 imagine as eliminated the membranous bones, the maxillae, the palatines, the vomer, 

 the pterygoid process, most of the great wing of the sphenoid, and the tympanic part 

 of the temporal. Figure 6 is drawn with the pharyngeal surface of the basioccipital 

 parallel to the plane of the paper. This is not the usual view of the adult skull, and 

 the latter must be rotated into the corresponding plane for comparison. 



The basioccipital and the basisphenoid present no new features of especial 

 interest not already considered, except the openings of the canal for the notochord. 

 The anterior opening where the notochord reenters the basal plate is probably 

 near or just back of the junction of the basioccipital and basisphenoid. The 

 exoccipital shows the large hypoglossal foramina; these are not covered by the 

 large condyles, as is the case in the adult. The condyles have scarcely begun to 

 develop, and the occipital is united to the atlas by dense mesenchyme without 

 articular surface. The tip of the occipital neural arch is quite distinct and sepa- 

 rated from the squama. The prominent transverse or jugular process lies just 

 caudal to the jugular foramen and, as in the adult, extends slightly lateral to the 

 outer border of the foramen. The process is more prominent than in the adult 

 and has more the character of the vertebral transverse processes. 



The immature condition of the inferior surface of the otic capsule and mastoid 

 region is accentuated by the fact that such structures as the carotid artery and 

 the facial nerve are not inclosed by cartilage, but lie partly in sulci beneath the 

 capsule. The internal carotid artery passes forward and medially in the mesen- 

 chyme beneath the cochlear part, with only the beginning of a groove formed by 

 precartilage, not indicated in this figure. It enters the cranial cavity between the 

 apex of the cochlear part and the alar process. The facial nerve, which enters the 

 otic capsule through the internal acoustic meatus, soon emerges again from 

 the facial foramen located in the anterior region of the capsular sulcus between 

 the cochlear and canalicular parts. The geniculate ganglion lies just outside this 

 foramen (figs. 12 and 13). The greater part of the facial nerve, which in the adult 

 is inclosed in bone, is extracapsular and passes backward and outward in a groove 

 on the under surface of the canalicular part. The groove lies between the fossa 

 incudis and the fenestra vestibuli or ovalis, and medial to the root of the styloid. 

 The cartilaginous auditory ossicles lie beneath the canalicular part (fig. 14). 



The serial relationship of the jugular process, the mastoid process, the upper 

 end of the styloid, and the incus is clearly indicated in figure 6, in which the 

 jugular, mastoid, root of the styloid, and fossa incudis are shown. It is impossible 

 to determine at present whether this has any phylogenetic significance or not. 

 In some of the embryos of about this stage there is a small separate cartilage in the 



