304 CAETILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



adult (including the lingulae) it will be seen that the ratio is 1 : 25, which corresponds 

 exactly to the ratio for the width of the basioccipital between the otic capsules in 

 the embryo and in the adult. One would naturally expect the two to grow at about 

 the same rate and, if this is true, then the alar process should not be looked upon as 

 ultimately forming part of the basisphenoid. Factors such as the possible pressure 

 of the otic capsules on this region of the basioccipital may, however, account for 

 the discrepancy, since the ratio for the distance between the hypoglossal fora- 

 minse (1 :15) is about the same as that for the basisphenoid, including in the embryo 

 the alar processes and in the adult the lingulae. 



DORSAL ASPECT. 



The most familiar aspect of the chondrocranium is the upper surface of the 

 base, shown in figures 1 and 2. These figures should be compared with a some- 

 what similar view of the upper surface of the base of the adult skull shown in figure 

 4, where the bone, ossified in cartilage, is colored blue. One is surprised, somewhat 

 startled in fact, by the general similarity between the cartilaginous skull of the 

 embryo and that part of the adult skull which is ossified in cartilage. The general 

 resemblance is even more marked in later stages when the chondrocranium is more 

 completely developed, as shown in the figures by Macklin and Hertwig of skulls 

 from embryos 40 and 80 mm. in length. Again, if the dorsal aspect of the base 

 of the entire skull, as shown in figure 3, is compared with that of the adult, new 

 resemblances will appear. In figure 3 is shown not only the cartilage, but also 

 the precartilage, blastema, and part of the dorsal membrane which enters into the 

 formation of the brain capsule. In comparing the adult skull with figures 1 and 

 3, it must be borne in mind that these figures are drawn with the basioccipital 

 horizontal and not inclined, as in the usual view of the adult skull or in the view of 

 the cartilaginous skull shown in figure 2. At the caudal end of the cartilaginous 

 skull is the basioccipital with the two lateral parts, or exoccipitals, one on each side. 

 The exoccipitals are continued into the occipital squamae or nuchal plates which are 

 not yet united in the middorsal line, so that the foramen magnum is incomplete. 

 The basioccipital is continuous in front into the basisphenoid, and from it the two 

 wings project on each side. The mesethmoid is continuous from the basisphenoid, 

 without line of demarcation, to the anterior end of the chondrocranium, and con- 

 nected with it are the nasal capsules. Lateral to the anterior half of the basi- 

 occipital and fused with it are the large otic capsules corresponding to the petrous 

 bones of the adult, each being separated from the exoccipital by the large jugular 

 foramen and the mastoid plate. 



The basioccipital consists of a flattened plate extending from the foramen 

 magnum to the basisphenoid. The caudal edge bordering the foramen magnum 

 is more deeply notched than in the adult. This notch is continued into a deep 

 groove on the cranial side of the cartilage. At the apex of the groove is the open- 

 ing where the notochord, after passing through the condensed mesenchyme which 

 fills the groove, enters the body of the occipital. 



