CARTILAGINOUS SKULL, OF A 21 MM. HUMAN EMBRYO. 311 



The basioccipital shows in its median sagittal section two parts a thin caudal 

 part lying beneath the notochord connecting the posterior part of the two lateral 

 thickened masses of the body from which the exoccipitals arise, and a long wedge- 

 shaped part in front of and above the notochord; this is thickest where it joins the 

 basisphenoid. The diagonal course of the notochord through the basioccipital 

 and its reentrance into the basal plate at or near the junction of the basioccipital 

 with the basiphenoid is also clearly shown. The three roots of the occipital neural 

 arch arise from the posterior half of the basioccipital and unite lateral to the 

 hypoglossal foramina into a tapering, rod-like cartilage. The serial relation ship 

 of the occipital neural arch with the cervical neural arches is very apparent in 

 figure 5. The dorsal tip is in line with the dorsal tips of the cervical neural arches. 

 The occipital squama is very prominent in this view; it appears to continue 

 upward from both the exoccipital and the otic capsule, and even to project in 

 front of the otic capsule. There is no line of demarcation between it and the 

 mastoid plate which encircles the posterior and upper edges of the canalicular 

 part of the otic capsule and thus no definite limits can be given to either cartil- 

 aginous area. The anterior part of this plate is usually called the parietal plate, 

 a term which we have already discussed and discarded, since it is not apparent 

 that it includes anything more than what might be embodied under the term 

 mastoid plate. In considering this region it should be borne in mind that figure 5 

 is drawn with the basioccipital horizontal. The large mastoid foramen (capsulo- 

 parietal fissure) lies at the bottom of the transverse sulcus and interrupts the 

 continuity of the mastoid plate. Above the region of the otic capsule the masto- 

 squamal plate is grooved for the endolymphatic sac. Whether this groove lies 

 on the mastoid or squamal part is uncertain. The fate of the various regions of 

 the masto-squamal plate will remain obscure until we have a more complete series 

 of stages. 



The surface of the otic capsule presented in this view corresponds to the 

 posterior surface of the petrous bone and faces the posterior cranial fossa. Its 

 diagonal position also corresponds to the adult when the latter is seen from the 

 same angle. The cochlear part is mostly above the level of the basioccipital, pro- 

 jecting but slightly below the level of the latter. The canalicular part shows the 

 position of part of the superior semicircular canal in front and of the posterior 

 canal in back. The endolymphatic sulcus serves to mark the position of the 

 common duct. 



The medial section of the basisphenoid extends only to the hypophysial canal 

 and a diagonal cut in front of this cuts off the orbital wing and extends to the edge 

 of the cartilage. It is impossible to determine the anterior and posterior limits 

 of the basisphenoid. The posterior limit is probably at or near the notochord 

 and the anterior limit somewhere near the line (*) . The body is biconcave in the 

 region of the sella turcica. Anteriorly, it narrows from side to side, as shown in 

 figure 1, and increases in its sagittal thickness, as shown in figure 5. The tem- 

 poral wing projects below the body and the orbital wing above. The mesethmoid 



