306 CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



of the other, and corresponds to the nuchal plate of the adult occipital squama, 

 which, as is well known, ossifies in cartilage. It seems quite unnecessary to intro- 

 duce into human embryology a term applied to a somewhat similar bridge of 

 cartilage, arching over the brain from one auditory capsule to the other, in the 

 cartilaginous skulls of amphibia and reptiles. The nuchal plate in the human 

 embryo is not homologous to the tectum synoticum of the reptiles, since in man it 

 is primarily continuous with the lateral parts of the occipital, and in reptiles with 

 the auditory capsules only. It would be better to avoid entirely the term tectum 

 posterius in human embryology, and to use the term nuchal plate, or squama, as 

 Levi has done, which at once gives to it its true significance in the higher vertebrates, 

 in man at least. 



The relations about the foramen magnum can be appreciated best by com- 

 paring figures 2 and 4. Figure 2 is drawn in the same position as the adult skull 

 and the squama is intact. In its growth around the central nervous system it 

 precedes that of the occipital neural arch and (as has been pointed out by Macklin) 

 fuses with its fellow of the opposite side to form the primitive foramen before the 

 occipital neural arches meet or nearly meet. 



The large jugular foramen lies between the exoccipital and the otic capsule, 

 as in the adult, extending laterally to the junction of the exoccipital with the 

 mastoid cartilage, and medially to the junction of the exoccipital with the otic 

 capsule. In the embryo and in the adult the same structures pass through the 

 foramen, but both the nerves and the vein are proportionally much larger in the 

 embryo. These structures have, however, approximately the same relations to 

 each other; the transverse sinus occupies the posterior and lateral part, the vagus 

 and the accessory nerves are just anterior and medial to it, and the glossopharyngeal 

 is still more anterior and medial and lies in the notch on the posterior edge of the 

 otic capsule. 



The temporal cartilage, as in the adult, has an otic capsule or petrous part 

 and a mastoid plate or mastoid part. The otic capsule forms one of the most 

 conspicuous features of the cartilaginous skull, especially at this stage, for at this 

 time the relatively small size of the occipital squama behind and of the orbital 

 wing of the sphenoid and the ethmoid cartilage in front gives even greater promi- 

 nence to the otic capsules than at a later stage when the squama and anterior end 

 of the chondrocranium are fully developed in cartilage. 



The otic capsule consists of a medial cochlear part fused with the lateral side of " 

 the anterior half of the basioccipital, and a lateral canalicular portion that forms part 

 of the lateral wall of the chondrocranium. The otic capsule is broader in propor- 

 tion to its length than the petrous bone. The internal acoustic meatus, the opening 

 of the aqueductus vestibuli, and the fossa subarcuata are all relatively enormous at 

 this stage. In the infant the fossa subarcuata also forms a relatively large depression. 



The direction of the axis of the otic capsule or petrous portion of the temporal 

 is very similar to that of the adult. The upper surface of the otic capsule, as 

 already noted, contains the internal acoustic meatus, the opening of the aque- 

 ductus vestibuli, and the large fossa subarcuata under the superior semicircular 



