CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 307 



canal. In the adult these openings are on the posterior surface of the petrous 

 bone. If the adult skull is rotated so that the cranial surface of the basioccipital 

 is horizontal, as in figure 1, of the embryonic chondrocranium, it will be seen that 

 the posterior surface of the petrous bone is directed upward and corresponds in 

 position to the upper surface of the otic capsule as seen in figure 1. In the embryo 

 one leaf of the tentorium cerebelli is attached to the anterior margin of the otic 

 capsule, and in the adult the tentorium is Likewise attached to the corresponding 

 margin of the petrous bone, so that the upper surface of the otic capsule faces the 

 posterior cranial fossa, as does the corresponding surface in the adult. All of the 

 mastoid plate (except its most anterior tip) and the squama of the occipital are pos- 

 terior to the tentorium and face the posterior cranial fossa, as do the mastoid 

 and nuchal plates of the adult. 



Surrounding the caudal and dorsal edges of the canalicular part, and con- 

 tinuous with it, is the mastoid plate or cartilage. This cartilage forms part of the 

 outer wall of the chondrocranium and is interrupted by a large mastoid foramen. 

 We have already noted the fusion of the mastoid plate with the exoccipital and occip- 

 ital squama. It is also grooved for the transverse sinus, and the latter covers 

 practically all of the inner surface of the cartilage. That portion of the mastoid 

 cartilage above the outer edge of the otic capsule corresponds, therefore, more 

 nearly to the upper part of the mastoid bone lying caudal to the upper part of the 

 outer end of the petrous bone. This becomes the more apparent when we consider 

 that the upper surface of the otic capsule, as seen in this aspect of the chondro- 

 cranium, presents both the internal acoustic meatus and the opening of the aque- 

 ductus vestibuli, and thus corresponds to the posterior surface of the petrous bone 

 and faces the posterior cranial fossa. 



The basisphenoid, which continues forward from the basioccipital without line 

 of demarcation, occupies about the same relative position as in the adult. The 

 dorsum sellse, although well marked, is but imperfectly developed and consists 

 mostly of precartilage and blastema, not indicated in figures 1 and 2 but shown 

 in figure 3. To this blastema is attached the medial part of the tentorium. Be- 

 tween the dorsum sellse and the tip of the otic capsule the abducens nerve passes 

 forward towards the orbit. In front of the dorsum sellse is the large shallow sella 

 turcica. The hypophysial canal is seen in the center of the sella and contains 

 remnants of the hypophysial stalk. Connected with the basisphenoid are the 

 temporal and orbital wings. The greater size of the orbital wings in the cartil- 

 aginous skull is exactly what we would expect when it is taken into consideration 

 that the part of the adult temporal wing which is ossified in cartilage is smaller 

 than the adult orbital wing ossified in cartilage. 



The temporal wings project laterally from the sides of the basisphenoid, just in 

 front of dorsum sellse and at a lower level than the floor of the sella turcica. These 

 wings form a small part of the very incomplete floor of the lateral parts of what 

 will later be the middle cranial fossa. Each lateral part of the middle cranial 

 fossa, although indicated here by the depressed area between the orbital wings 

 and the otic capsules, is entirely filled by the lower end of the embryonic tentorium 



