310 CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



In the cartilaginous skull the optic foramen is incomplete but, as shown in 

 figure 3, is completed by the precartilage extending from the tip of the orbital wing 

 to the anterior part of the body of the sphenoid. 



In figure 1 it can be seen how the anterior end of the basisphenoid rapidly 

 narrows and continues into the mesethmoid. Both in figures 1 and 2 the dorsal 

 thin edge of the mesethmoid can be seen; it corresponds in the adult to that part 

 of the mesethmoid which bisects the posterior part of the cribriform plate. The 

 mesethmoid extends into the large crista galli, which consists mostly of precarti- 

 lage. In figure 3 the position of the future cribriform plate is indicated by the large 

 olfactory foramina, one on either side of the short dorsal edge of the mesethmoid. 

 On either side of the mesethmoid, and connected with the anterior edge in front of 

 the crista galli, are the lateral wings of the nasal capsules. As seen in figures 1 

 and 2, the posterior edge of each capsule is presented. 



The anterior cranial fossa is not evident in the cartilaginous skull. In the 

 combined cartilaginous and membranous skull shown in figure 3 its limits are 

 clearly indicated. The floor of the fossa is formed by the precartilage of the orbital 

 wing, frontal blastema, and dorsal membrane, and in the center are the mesethmoid 

 and crista galli and the olfactory and ethmoid foramina. Figure 3 shows not 

 only the floor but also a considerable extent of the anterior wall formed by the 

 dorsal membrane. 



MEDIAN SAGITTAL ASPECT. 



In order to compare the view from the median sagittal plane shown in figure 

 5 with a similar view of the adult skull, the latter should be rotated so that the 

 cranial surface of the basioccipital is horizontal. In the median sagittal section 

 of the adult skull before me the pharyngeal edge of the basioccipital measures 26 

 mm., the basisphenoid 24 mm., and the upper edge of the vomer (from the basis- 

 phenoid to the anterior nasal spine of the maxilla) 54 mm. The latter distance 

 corresponds to the ventral edge of the mesethmoid. In the cartilaginous skull the 

 corresponding distances are 2 mm., 1.6 mm., and 1.8 mm. The ratios, then, are 

 respectively 1:13, 1:15 and 1:30. The basisphenoid is thus proportionally shorter 

 in the embryo at this stage than the basioccipital, and the mesethmoid much more 

 so. This is what one would expect to find in the less differentiated regions at the 

 anterior end of the skull. 



The angles which the pharyngeal surface of the basioccipital make with the 

 same surface of the basisphenoid, and the latter with the mesethmoid, are prac- 

 tically identical with those found in the adult. Since the edge of the basisphenoid 

 is concave, a straight line connecting the two ends was used. It is interesting 

 to note in this connection that the angle made by the cranial surface of the basi- 

 occipital, projected sagittally on to a line parallel with the upper edge of the zygo- 

 matic arch, is almost exactly the same in this embryo as in the adult. Such 

 measurements serve to indicate that the cartilaginous skull is laid down from the 

 very beginning on much the same plan as that part of the adult skull ossified in 

 cartilage. 



